diff --git a/mkdocs.yml b/mkdocs.yml
index c81a99bc26..ef8b344d4d 100644
--- a/mkdocs.yml
+++ b/mkdocs.yml
@@ -48,6 +48,7 @@ nav:
           - Quantitative MRI: appendices/qmri.md
           - Arterial Spin Labeling: appendices/arterial-spin-labeling.md
           - Cross modality correspondence: appendices/cross-modality-correspondence.md
+          - Phenotypic data guidelines: appendices/phenotype.md
       - Changelog: CHANGES.md
   - The BIDS Website:
       - Getting started: https://bids.neuroimaging.io/getting_started/
diff --git a/src/appendices/phenotype.md b/src/appendices/phenotype.md
new file mode 100644
index 0000000000..a6edfdfc70
--- /dev/null
+++ b/src/appendices/phenotype.md
@@ -0,0 +1,412 @@
+# Tabular phenotypic data guidelines
+
+This appendix is a collection of guidelines and examples
+for curating well-organized tabular phenotypic data.
+
+## Guidelines
+
+These guidelines are intended to improve the organization and clarity of
+tabular phenotypic data like the participants file, sessions file,
+and phenotypic and assessment data.
+
+They are recommendations and are by default ignored during validation.
+You can make them mandatory during validation by setting the
+[`AdditionalValidation` key](../modality-agnostic-files/dataset-description.md#additional-validation)
+to contain `"Phenotype"` in the `dataset_description.json`.
+
+### 1. Aggregate data across sessions using `"IndexColumns"`
+
+In multi-session datasets,
+aggregate phenotypic and assessment data across all sessions
+into one tabular tab-separated value (TSV) file per measurement tool.
+In order to aggregate, you MUST use the `"IndexColumns"` list/array field
+in the corresponding JavaScript Object Notation (JSON) sidecar file.
+There are two examples of this usage [below in this appendix](#examples).
+Store each of the TSV and JSON files in the `/phenotype` directory
+using the file-naming template `/phenotype/tool-<ToolName>_phenotype.tsv`.
+Read the [phenotypic and assessment data section](../modality-agnostic-files/phenotypic-and-assessment-data.md)
+for further explanation of how to use `"IndexColumns"`
+to aggregate longitudinal or multi-session tabular phenotypic data.
+
+### 2. Always pair tabular data with data dictionaries
+
+Tabular phenotypic data MUST be prepared as one pair of a tabular file
+in TSV format and a corresponding data dictionary in JSON format.
+See the [Tabular files section](../common-principles.md#tabular-files) for more information.
+
+### 3. Add `MeasurementToolMetadata` to each tabular phenotypic measurement tool
+
+Whenever possible, it is RECOMMENDED to add `MeasurementToolMetadata` to
+each `phenotype/<measurement_tool_name>.json` data dictionary.
+This improves reusability and provides clarity about the measurement tool.
+See [`MeasurementToolMetadata` in the glossary](../glossary.md#measurementtoolmetadata-metadata) for more.
+
+### 4. Ensure minimal annotation for phenotypic and assessment data
+
+In multi-session phenotypic and assessment data,
+each measurement tool SHOULD have an independent
+aggregated data TSV file in which the user collects all subjects, sessions,
+and/or runs of data as one entry per row
+(with a row defined by the smallest unit of acquisition).
+This also means the user MUST use the `"IndexColumns"` field
+in each JSON sidecar for multi-session data.
+Some common index columns are:
+`participant_id`, `session_id`, `run_id`, and `acq_time`.
+
+<!-- This block generates a columns table.
+The definitions of these fields can be found in
+  src/schema/rules/tabular_data/*.yaml
+and a guide for using macros can be found at
+ https://github.com/bids-standard/bids-specification/blob/master/macros_doc.md
+-->
+{{ MACROS___make_columns_table("modality_agnostic.Phenotypes") }}
+
+Furthermore, if you add a `session_id` index column to any tabular phenotypic data,
+you MUST introduce a session directory to the imaging data,
+even if only one imaging session has been created.
+And vice versa, if imaging data has session directories,
+all imaging data and tabular phenotypic data MUST have sessions.
+
+This produces files in which same-participant entries can take up as many rows as needed
+according to the smallest unit of acquisition.
+
+### 5. Use a demographics file for multi-session data
+
+If there is more than one session for any one participant, then
+it is REQUIRED to provide a demographics file in the `/phenotype` directory
+named as `/phenotype/tool-Demographics_phenotype.tsv`
+using the `"IndexColumns"` JSON sidecar field.
+It is RECOMMENDED to store the `age` column for multi-session datasets
+in this demographics file to record participant age for every session
+on their own rows.
+
+### 6. Record acquisition time of all sessions with `acq_time`
+
+It is RECOMMENDED to store acquisition time[<sup>2</sup>](#footnotes)
+for tabular phenotypic data and store the time of acquisition of each row
+inside a column named `acq_time` in the demographics file.
+This is consistent with how acquisition time is recorded for MRI data
+and other time-sensitive measurements (for example systolic blood pressure).
+
+## Summary
+
+This appendix described guidelines for best tabular phenotypic data.
+In summary, it is RECOMMENDED to always use the participants file
+and separate files by measurement instrument in
+the phenotypic and assessment data directory,
+since they each collect different information.
+If you have multi-session data, then follow the aggregation guidelines above.
+
+## Examples
+
+What follows are a few common use case examples for tabular phenotypic files.
+
+### 1 participant session with both non-tabular and tabular phenotypic data
+
+File tree
+
+<!-- This block generates a file tree.
+A guide for using macros can be found at
+ https://github.com/bids-standard/bids-specification/blob/master/macros_doc.md
+-->
+{{ MACROS___make_filetree_example(
+   {
+   "phenotype": {
+      "tool-Measurements_phenotype.json": "",
+      "tool-Measurements_phenotype.tsv": "",
+      },
+   "sub-01": {
+      "anat": {
+         "sub-01_T1w.json": "",
+         "sub-01_T1w.nii.gz": "",
+         }
+      }
+   }
+) }}
+
+Contents of `phenotype/tool-Measurements_phenotype.tsv`
+
+```tsv
+participant_id	measurement_1	measurement_2
+sub-01	value1	value2
+```
+
+### 1 participant with 2 sessions, where 1 session is only tabular phenotype and the other is only imaging
+
+With only one imaging and one phenotypic session each in this example you might want
+to merge both imaging and phenotypic data under one session. But it is more correct to
+have separate sessions for the imaging and phenotypic data, especially if
+the sessions were collected days, weeks, or months apart. You can denote all of
+`participant_id`, `session_id`, and `acq_time` in the `tool-Measurements_phenotype.tsv` file
+and note `session_id` `Levels` in the `tool-Measurements_phenotype.json` sidecar.
+Below are a CORRECT and an INCORRECT example of prepared data following these guidelines.
+
+#### CORRECT
+
+File tree
+
+<!-- This block generates a file tree.
+A guide for using macros can be found at
+ https://github.com/bids-standard/bids-specification/blob/master/macros_doc.md
+-->
+{{ MACROS___make_filetree_example(
+   {
+   "phenotype": {
+      "tool-Measurements_phenotype.json": "",
+      "tool-Measurements_phenotype.tsv": "",
+      },
+   "sub-01": {
+      "ses-MRI": {
+         "anat": {
+            "sub-01_ses-MRI_T1w.json": "",
+            "sub-01_ses-MRI_T1w.nii.gz": "",
+            }
+         }
+      }
+   }
+) }}
+
+Contents of `phenotype/tool-Measurements_phenotype.tsv`
+
+```tsv
+participant_id	session_id	acq_time	measurement_1	measurement_2
+sub-01	ses-pheno	2001-01-01T12:05:00	value1	value2
+sub-01	ses-MRI	2001-03-01T13:14:00	n/a	n/a
+```
+
+Contents of `phenotype/tool-Measurements_phenotype.json`
+
+```json
+{
+    "IndexColumns": [
+        "participant_id",
+        "session_id"
+    ],
+    "participant_id": {
+        "Description": "Participant identifier"
+    },
+    "session_id": {
+        "Description": "Session identifier",
+        "Levels": {
+            "ses-pheno": "Phenotype-only session",
+            "ses-MRI": "MRI-only session"
+        }
+    },
+    "acq_time": {
+        "Description": "When the data acquisition started"
+    },
+    "measurement_1": {
+        "Description": "A first measurement taken at a phenotypic session"
+    },
+    "measurement_2": {
+        "Description": "A second measurement taken at a phenotypic session"
+    }
+}
+```
+
+#### INCORRECT
+
+File tree
+
+<!-- This block generates a file tree.
+A guide for using macros can be found at
+ https://github.com/bids-standard/bids-specification/blob/master/macros_doc.md
+-->
+{{ MACROS___make_filetree_example(
+   {
+   "phenotype": {
+      "tool-Measurements_phenotype.json": "",
+      "tool-Measurements_phenotype.tsv": "",
+      },
+   "sub-01": {
+      "anat": {
+         "sub-01_T1w.json": "",
+         "sub-01_T1w.nii.gz": "",
+         }
+      }
+   }
+) }}
+
+Contents of `phenotype/tool-Measurements_phenotype.tsv`
+
+```tsv
+participant_id	measurement_1	measurement_2
+sub-01	value1	value2
+```
+
+A session directory MUST be present in the participant's directory and
+the `session_id` column MUST be present
+in `phenotype/tool-Measurements_phenotype.tsv`,
+and the `"IndexColumns"` of `participant_id` and `session_id` MUST be present
+in `phenotype/tool-Measurements_phenotype.json`.
+Sessions must be used consistently for the combination of tabular and
+non-tabular phenotypic data.
+
+### 2 participants with a mix of tabular phenotypic data and imaging sessions
+
+In this example, participants acquired both
+a phenotypic measurement tool and an MRI during `ses-MRI1`.
+`sub-01` has a `ses-MRI2` with no phenotypic measurement tool acquired
+and `sub-02` has a `ses-pheno` where no MRI was acquired.
+
+File tree
+
+<!-- This block generates a file tree.
+A guide for using macros can be found at
+ https://github.com/bids-standard/bids-specification/blob/master/macros_doc.md
+-->
+{{ MACROS___make_filetree_example(
+   {
+   "phenotype": {
+      "tool-Measurements_phenotype.json": "",
+      "tool-Measurements_phenotype.tsv": "",
+      },
+   "sub-01": {
+      "ses-MRI1": {
+         "anat": {
+            "sub-01_ses-MRI1_T1w.json": "",
+            "sub-01_ses-MRI1_T1w.nii.gz": "",
+            }
+         },
+      "ses-MRI2": {
+         "anat": {
+            "sub-01_ses-MRI2_T1w.json": "",
+            "sub-01_ses-MRI2_T1w.nii.gz": "",
+            }
+         }
+      },
+   "sub-02": {
+      "ses-MRI1": {
+         "anat": {
+            "sub-02_ses-MRI1_T1w.json": "",
+            "sub-02_ses-MRI1_T1w.nii.gz": "",
+            }
+         }
+      }
+   }
+) }}
+
+Contents of `phenotype/tool-Measurements_phenotype.tsv`
+
+```tsv
+participant_id	session_id	acq_time	measurement_1	measurement_2
+sub-01	ses-MRI1	2001-01-01T11:12:00	value1	value2
+sub-01	ses-MRI2	2001-07-01T13:14:00	n/a	n/a
+sub-02	ses-MRI1	2001-01-181T15:16:00	value3	value4
+sub-02	ses-pheno	2001-02-20T12:05:00	value5	value6
+```
+
+### 3 participants with 3 different kinds of sessions among them
+
+The `ses-baseline` session collects an MRI and tabular phenotypic data.
+
+File tree
+
+<!-- This block generates a file tree.
+A guide for using macros can be found at
+ https://github.com/bids-standard/bids-specification/blob/master/macros_doc.md
+-->
+{{ MACROS___make_filetree_example(
+   {
+   "participants.json": "",
+   "participants.tsv": "",
+   "phenotype": {
+      "tool-Demographics_phenotype.json": "",
+      "tool-Demographics_phenotype.tsv": "",
+      "tool-Survey_phenotype.json": "",
+      "tool-Survey_phenotype.tsv": "",
+      },
+   "sub-01": {
+      "ses-baseline/": "",
+      "ses-followupMRI/": "",
+      },
+   "sub-02": {
+      "ses-baseline/": "",
+      },
+   "sub-03": {
+      "ses-baseline/": "",
+      "ses-followupMRI/": "",
+      }
+   }
+) }}
+
+Contents of `participants.tsv`.
+Unchanging participant properties belong here.
+
+```tsv
+participant_id	sex
+sub-01	M
+sub-02	F
+sub-03	F
+```
+
+Contents of `phenotype/tool-Demographics_phenotype.tsv`.
+Participant properties that can change
+from session to session belong here especially.
+
+```tsv
+participant_id	session_id	acq_time	age	gender	race	household_income
+sub-01	ses-baseline	2001-01-01T12:05:00	10	3	4	5
+sub-01	ses-followupMRI	2001-07-01T13:33:00	10	3	4	5
+sub-01	ses-interview	2002-01-01T11:21:00	11	4	4	6
+sub-02	ses-baseline	2001-04-01T11:01:00	9	1	3	3
+sub-02	ses-interview	2002-04-01T14:08:00	10	1	7	3
+sub-03	ses-baseline	2001-09-01T11:45:00	11	2	10	4
+sub-03	ses-followupMRI	2002-03-01T12:17:00	12	5	10	4
+```
+
+Partial contents of `phenotype/tool-Demographics_phenotype.json`.
+Note how the `session_id` `Levels` are clearly described
+and how `"IndexColumns"` is present.
+
+```json
+{
+    "IndexColumns": [
+        "participant_id",
+        "session_id"
+    ],
+    "participant_id": {
+        "Description": "Participant identifier"
+    },
+    "session_id": {
+        "Description": "Session identifier",
+        "Levels": {
+            "ses-baseline": "Baseline visit for MRI and assessments",
+            "ses-followupMRI": "6-months after baseline MRI follow-up",
+            "ses-interview": "1-year after baseline in-person follow-up"
+        }
+    },
+    "acq_time": {
+        "Description": "When the data acquisition started"
+    }
+}
+```
+
+Contents of `phenotype/tool-Survey_phenotype.tsv`.
+Note how `sub-03` does not have a row for `ses-interview`
+because that session was not collected and is absent above
+in the `phenotype/tool-Demographics_phenotype.tsv` file as well.
+
+```tsv
+participant_id	session_id	question_1	question_2	question_3
+sub-01	ses-baseline	A	2	no
+sub-01	ses-interview	A	3	yes
+sub-02	ses-baseline	A	2	no
+sub-02	ses-interview	B	1	unsure
+sub-03	ses-baseline	B	3	no
+```
+
+For more complete examples, see the `pheno00*`
+[bids-examples on GitHub](https://github.com/bids-standard/bids-examples/).
+
+## Footnotes
+
+<sup>1</sup> A session is any logical grouping of imaging and behavioral data consistent
+across participants. Session can (but doesn't have to) be synonymous to a visit
+in a longitudinal study. In situations where different data types are obtained over
+several visits (for example fMRI on one day followed by DWI the day after)
+those can still be grouped in one session. Refer to the
+[definition of session](../glossary.md#session-entities) for more details.
+
+<sup>2</sup> Datetime format and the anonymization procedure are
+described in [Units](../common-principles.md#units).
diff --git a/src/common-principles.md b/src/common-principles.md
index 403c7f702b..e421cfce60 100644
--- a/src/common-principles.md
+++ b/src/common-principles.md
@@ -510,7 +510,7 @@ NIfTI header.
 
 ### Tabular files
 
-Tabular data MUST be saved as plain-text, tab-delimited values (TSV) files
+Tabular data MUST be saved as plain-text, tab-separated values (TSV) files
 (with [extension `.tsv`](glossary.md#tsv-extensions)),
 that is, [CSV files](https://en.wikipedia.org/wiki/Comma-separated_values) where commas are replaced by tab characters.
 Tabs MUST be true tab characters and MUST NOT be a series of space characters.
@@ -561,7 +561,7 @@ onset	duration	response_time	trial_type	trial_extra
     are not part of the tabular data file's content.
 
 Tabular files MAY be optionally accompanied by a simple data dictionary
-in the form of a JSON [object](https://www.json.org/json-en.html)
+in the form of a [JSON object](https://www.json.org/json-en.html)
 within a JSON file.
 The JSON files containing the data dictionaries MUST have the same name as
 their corresponding tabular files but with `.json` extensions.
diff --git a/src/longitudinal-and-multi-site-studies.md b/src/longitudinal-and-multi-site-studies.md
index 5623bec305..5c79c6b0ae 100644
--- a/src/longitudinal-and-multi-site-studies.md
+++ b/src/longitudinal-and-multi-site-studies.md
@@ -109,3 +109,10 @@ underscores are not allowed in subject labels.
 
 In case of studies such as "Traveling Human Phantom" it is possible to incorporate site within session label.
 For example `sub-human1/ses-NUY`, `sub-human1/ses-MIT`, `sub-phantom1/ses-NUY`, `sub-phantom1/ses-MIT` and so on.
+
+## On tabular phenotypic data across sessions
+
+Read the [tabular phenotypic data guidelines appendix](appendices/phenotype.md)
+or the [phenotypic and assessment data section](modality-agnostic-files/phenotypic-and-assessment-data.md)
+for an explanation of how to use `"IndexColumns"`
+to aggregate longitudinal or multi-session tabular phenotypic data.
diff --git a/src/metaschema.json b/src/metaschema.json
index 7d5b83ebf5..4a843788d4 100644
--- a/src/metaschema.json
+++ b/src/metaschema.json
@@ -326,8 +326,11 @@
                   "type": "object",
                   "properties": {
                     "issue": {
-                      "allOf": [{ "$ref": "#/definitions/ruleTypes/issue" }],
-                      "required": ["level"]
+                      "allOf": [
+                        { "$ref": "#/definitions/ruleTypes/issue" },
+                        { "required": ["level"] }
+                      ],
+                      "unevaluatedProperties": false
                     },
                     "selectors": {
                       "$ref": "#/definitions/ruleTypes/expressionList"
@@ -473,17 +476,18 @@
           "type": "object",
           "patternProperties": {
             "^[a-zA-Z0-9_]+$": {
-              "type": "object",
-              "properties": {
-                "code": { "type": "string" },
-                "message": { "type": "string" },
-                "level": { "enum": ["error", "warning"] },
-                "selectors": {
-                  "$ref": "#/definitions/ruleTypes/expressionList"
+              "allOf": [
+                { "$ref": "#/definitions/ruleTypes/issue" },
+                {
+                  "properties": {
+                    "selectors": {
+                      "$ref": "#/definitions/ruleTypes/expressionList"
+                    }
+                  },
+                  "required": ["level"]
                 }
-              },
-              "required": ["message", "level"],
-              "additionalProperties": false
+              ],
+              "unevaluatedProperties": false
             }
           },
           "additionalProperties": false
@@ -595,7 +599,10 @@
               "level": { "$ref": "#/definitions/enums/requirement_level" },
               "level_addendum": { "type": "string" },
               "description_addendum": { "type": "string" },
-              "issue": { "$ref": "#/definitions/ruleTypes/issue" }
+              "issue": {
+                "$ref": "#/definitions/ruleTypes/issue",
+                "unevaluatedProperties": false
+              }
             },
             "required": ["level"],
             "additionalProperties": false
@@ -606,11 +613,13 @@
         "type": "object",
         "properties": {
           "code": { "type": "string" },
+          "subCode": { "type": "string" },
+          "level": { "enum": ["error", "warning"] },
+          "location": { "type": "string" },
           "message": { "type": "string" },
-          "level": { "enum": ["error", "warning"] }
+          "suggestion": { "type": "string" }
         },
-        "required": ["code", "message"],
-        "additionalProperties": false
+        "required": ["code", "message"]
       },
       "expressionList": {
         "type": "array",
diff --git a/src/modality-agnostic-files/data-summary-files.md b/src/modality-agnostic-files/data-summary-files.md
index c615196b5c..cf4884b879 100644
--- a/src/modality-agnostic-files/data-summary-files.md
+++ b/src/modality-agnostic-files/data-summary-files.md
@@ -53,6 +53,9 @@ to date of birth.
 
 ```JSON
 {
+    "participant_id": {
+        "Description": "participant identifier"
+    },
     "age": {
         "Description": "age of the participant",
         "Units": "year"
@@ -81,6 +84,24 @@ to date of birth.
 }
 ```
 
+It is RECOMMENDED to use the `age` column to record participant age.
+This reduces data duplication across tabular data files. The `Units` of `age`
+do not have to be years so long as the units of the age
+are written in `participants.json`.
+Consider participant privacy or study objectives when selecting
+the `Units` of `age` or the accuracy of `age` data.
+
+There are two methods to record `age` in longitudinal or multi-session data sets.
+Choose what makes the most sense for your dataset's expected users.
+The first method is to aggregate into a single file in the phenotypic and assessment data folder
+using the `"IndexColumns"` field in the sidecar JSON.
+Read the [tabular phenotypic data guidelines appendix](../appendices/phenotype.md)
+or the [phenotypic and assessment data section](phenotypic-and-assessment-data.md)
+for an explanation of how to use `"IndexColumns"`
+to aggregate longitudinal or multi-session tabular phenotypic data.
+The second method is to segregate `age` into participant's session files.
+Read the [sessions file section](#sessions-file) below for further explanation.
+
 ## Samples file
 
 Template:
@@ -201,19 +222,36 @@ meg/sub-control01_task-rest_split-02_meg.nii.gz	1877-06-15T12:15:27
 
 Template:
 
-```Text
-sub-<label>/
-    sub-<label>_sessions.tsv
-```
+<!-- This block generates a file tree.
+A guide for using macros can be found at
+ https://github.com/bids-standard/bids-specification/blob/master/macros_doc.md
+-->
+{{ MACROS___make_filetree_example(
+   {
+   "sub-<label>": {
+      "sub-<label>_sessions.tsv": "",
+      "[sub-<label>_sessions.json]": "",
+      }
+   }
+) }}
 
 Optional: Yes
 
-In case of multiple sessions there is an option of adding additional
-`sessions.tsv` files describing variables changing between sessions.
-In such case one file per participant SHOULD be added.
-These files MUST include a `session_id` column and describe each session by one and only one row.
-Column names in `sessions.tsv` files MUST be different from group level participant key column names in the
-[`participants.tsv` file](./data-summary-files.md#participants-file).
+In case of multiple sessions there is an option of adding an additional
+sessions file describing each session and variables changing between sessions.
+Column names in sessions files MUST be different from participant key column names in
+the [participants file](#participants-file).
+
+These files MUST start with a `session_id` column
+and describe each session by one and only one row.
+`sub-<label>/sub-<label>_sessions.tsv` example:
+
+```tsv
+session_id	acq_time	systolic_blood_pressure
+ses-predrug	2009-06-15T13:45:30	120
+ses-postdrug	2009-06-16T13:45:30	100
+ses-followup	2009-06-17T13:45:30	110
+```
 
 <!-- This block generates a columns table.
 The definitions of these fields can be found in
@@ -223,11 +261,44 @@ and a guide for using macros can be found at
 -->
 {{ MACROS___make_columns_table("modality_agnostic.Sessions") }}
 
-`_sessions.tsv` example:
+`sub-<label>/sub-<label>_sessions.json` example:
 
-```tsv
-session_id	acq_time	systolic_blood_pressure
-ses-predrug	2009-06-15T13:45:30	120
-ses-postdrug	2009-06-16T13:45:30	100
-ses-followup	2009-06-17T13:45:30	110
+```JSON
+{
+    "session_id": {
+        "Description": "Session identifier",
+        "Levels": {
+            "ses-predrug": "session before drug administration",
+            "ses-postdrug": "session after drug administration",
+            "ses-followup": "follow-up session"
+        }
+    },
+    "acq_time": {
+        "Description": "Acquisition time of the session"
+    },
+    "systolic_blood_pressure": {
+        "Description": "Systolic blood pressure measured at the beginning of the session in mmHg"
+    }
+}
 ```
+
+Read the [tabular phenotypic data guidelines appendix](../appendices/phenotype.md)
+or the [phenotypic and assessment data section](phenotypic-and-assessment-data.md)
+for an explanation of how to use `"IndexColumns"`
+to aggregate longitudinal or multi-session tabular phenotypic data.
+
+### Additional validation
+
+When the [`AdditionalValidation` key](dataset-description.md#additional-validation)
+contains `"Phenotype"` in the `dataset_description.json`,
+the following tabular phenotypic data guidelines
+apply to sessions files:
+
+-   [5.](../appendices/phenotype.md#5-use-a-demographics-file-for-multi-session-data)
+    Use a demographics file for multi-session data
+
+-   [6.](../appendices/phenotype.md#6-record-acquisition-time-of-all-sessions-with-acq_time)
+    Record acquisition time of all sessions with `acq_time`
+
+To read more about the guidelines for tabular phenotypic data and examples,
+see the [tabular phenotypic data guidelines appendix](../appendices/phenotype.md).
diff --git a/src/modality-agnostic-files/dataset-description.md b/src/modality-agnostic-files/dataset-description.md
index 769afb0e9f..01c8118b85 100644
--- a/src/modality-agnostic-files/dataset-description.md
+++ b/src/modality-agnostic-files/dataset-description.md
@@ -145,6 +145,19 @@ Example:
 }
 ```
 
+### Additional validation
+
+The `AdditionalValidation` key MAY be used to opt into additional validation
+to be performed on the dataset beyond standard BIDS validation.
+The value of this field is an array of strings,
+each of which MUST be the name of a supported additional validation to be performed.
+
+The currently supported values are:
+
+| **Value**     | **Description**                                                                                                        |
+| ------------- | ---------------------------------------------------------------------------------------------------------------------- |
+| `"Phenotype"` | Stricter validation for tabular phenotypic data, as described in the [phenotype appendix](../appendices/phenotype.md). |
+
 ## `README`
 
 <!-- This block generates a file tree.
diff --git a/src/modality-agnostic-files/phenotypic-and-assessment-data.md b/src/modality-agnostic-files/phenotypic-and-assessment-data.md
index d876a38ee7..58042e6f2f 100644
--- a/src/modality-agnostic-files/phenotypic-and-assessment-data.md
+++ b/src/modality-agnostic-files/phenotypic-and-assessment-data.md
@@ -4,8 +4,8 @@ Template:
 
 ```Text
 phenotype/
-    <measurement_tool_name>.tsv
-    <measurement_tool_name>.json
+    [tool-]<MeasurementToolName>[_phenotype].tsv
+    [tool-]<MeasurementToolName>[_phenotype].json
 ```
 
 Optional: Yes
@@ -14,7 +14,7 @@ If the dataset includes multiple sets of participant level measurements (for
 example responses from multiple questionnaires) they can be split into
 individual files separate from `participants.tsv`.
 
-Each of the measurement files MUST be kept in a `/phenotype` directory placed
+Each of the measurement tool files MUST be kept in a `/phenotype` directory placed
 at the root of the BIDS dataset and MUST end with the `.tsv` extension.
 Filenames SHOULD be chosen to reflect the contents of the file.
 For example, the "Adult ADHD Clinical Diagnostic Scale" could be saved in a file
@@ -24,9 +24,21 @@ The files can include an arbitrary set of columns, but one of them MUST be
 `participant_id` and the entries of that column MUST correspond to the subjects
 in the BIDS dataset and `participants.tsv` file.
 
-As with all other tabular data, the additional phenotypic information files
-MAY be accompanied by a JSON file describing the columns in detail
+<!-- This block generates a columns table.
+The definitions of these fields can be found in
+  src/schema/rules/tabular_data/*.yaml
+and a guide for using macros can be found at
+ https://github.com/bids-standard/bids-specification/blob/master/macros_doc.md
+-->
+{{ MACROS___make_columns_table("modality_agnostic.Phenotypes") }}
+
+As with all other tabular data, the additional tabular phenotypic data
+MAY be accompanied by a JSON data dictionary file describing the columns in detail
 (see [Tabular files](../common-principles.md#tabular-files)).
+When the [`AdditionalValidation` key](dataset-description.md#additional-validation)
+contains `"Phenotype"` in the `dataset_description.json`,
+then the additional tabular phenotypic data
+MUST be accompanied by a JSON data dictionary file.
 
 In addition to the column descriptions, the JSON file MAY contain the following fields:
 
@@ -38,20 +50,36 @@ and a guide for using macros can be found at
 -->
 {{ MACROS___make_metadata_table(
    {
+      "IndexColumns": "OPTIONAL",
       "MeasurementToolMetadata": "OPTIONAL",
       "Derivative": "OPTIONAL",
    }
 ) }}
 
+The `"IndexColumns"` field defines a "joint index".
+This means there MAY be more than one row per `participant_id`,
+but there MUST be no more than one unique combination of
+the joint indices defined in the list among all rows.
+
 As an example, consider the contents of a file called
-`phenotype/acds_adult.json`:
+`phenotype/tool-ACDSAdult_phenotype.json` in a multi-session dataset:
 
 ```JSON
 {
+  "IndexColumns": [
+    "participant_id",
+    "session_id"
+  ],
   "MeasurementToolMetadata": {
     "Description": "Adult ADHD Clinical Diagnostic Scale V1.2",
     "TermURL": "https://www.cognitiveatlas.org/task/id/trm_5586ff878155d"
   },
+  "participant_id": {
+    "Description": "BIDS participant identifier, of the format sub-<label>"
+  },
+  "session_id": {
+    "Description": "BIDS session identifier, of the format ses-<label>"
+  },
   "adhd_b": {
     "Description": "B. CHILDHOOD ONSET OF ADHD (PRIOR TO AGE 7)",
     "Levels": {
@@ -69,14 +97,43 @@ As an example, consider the contents of a file called
 }
 ```
 
-Please note that in this example `MeasurementToolMetadata` includes information
-about the questionnaire and `adhd_b` and `adhd_c_dx` correspond to individual
-columns.
+Please note in this example
+there MUST be no more than one unique combination of
+the joint indices of `participant_id` and `session_id`.
+Also in the above example, `MeasurementToolMetadata`
+includes information about the questionnaire,
+and `participant_id`, `session_id`, `adhd_b`, and `adhd_c_dx`
+correspond to individual columns in the TSV data.
+
+For TSV examples, read the
+[tabular phenotypic data guidelines appendix](../appendices/phenotype.md#examples).
 
 In addition to the keys available to describe columns in all tabular files
 (`LongName`, `Description`, `Levels`, `Units`, and `TermURL`) the
 `participants.json` file as well as phenotypic files can also include column
 descriptions with a `Derivative` field that, when set to true, indicates that
-values in the corresponding column is a transformation of values from other
+values in the corresponding column are a transformation of values from other
 columns (for example a summary score based on a subset of items in a
 questionnaire).
+
+## Additional validation
+
+When the [`AdditionalValidation` key](dataset-description.md#additional-validation)
+contains `"Phenotype"` in the `dataset_description.json`,
+the following tabular phenotypic data guidelines
+apply to phenotypic and assessment data.
+
+-   [1.](../appendices/phenotype.md#1-aggregate-data-across-sessions-using-indexcolumns)
+    Aggregate data across sessions using `"IndexColumns"`
+
+-   [2.](../appendices/phenotype.md#2-always-pair-tabular-data-with-data-dictionaries)
+    Always pair tabular data with data dictionaries
+
+-   [3.](../appendices/phenotype.md#3-add-measurementtoolmetadata-to-each-tabular-phenotypic-measurement-tool)
+    Add `MeasurementToolMetadata` to each tabular phenotypic measurement tool
+
+-   [4.](../appendices/phenotype.md#4-ensure-minimal-annotation-for-phenotypic-and-assessment-data)
+    Ensure minimal annotation for phenotypic and assessment data
+
+To read more about the guidelines for tabular phenotypic data and examples,
+see the [tabular phenotypic data guidelines appendix](../appendices/phenotype.md).
diff --git a/src/schema/README.md b/src/schema/README.md
index c0da0fa55e..6edc43e19f 100644
--- a/src/schema/README.md
+++ b/src/schema/README.md
@@ -286,6 +286,7 @@ The following functions should be defined by an interpreter:
 | `allequal(a: array, b: array) -> bool`            | `true` if arrays have the same length and paired elements are equal                                                                       | `allequal(sorted(columns.onset, "numeric"), columns.onset)` | True if the array columns.onset is sorted numerically.                         |
 | `count(arg: array, val: any) -> int`              | Number of elements in an array equal to `val`                                                                                             | `count(columns.type, "EEG")`                                | The number of times "EEG" appears in the column "type" of the current TSV file |
 | `exists(arg: str \| array, rule: str) -> int`     | Count of files in an array that exist in the dataset. String is array with length 1. See following section for the meanings of rules.     | `exists(sidecar.IntendedFor, "subject")`                    | True if all files in `IntendedFor` exist, relative to the subject directory.   |
+| `glob(arg: str) -> array`                         | An array of files in the dataset that match the `arg` string. Strings may include single and double stars (`*`, `**`).                    | `glob("sub-*")`                                             | Non-empty array if any subject directories are present in the dataset.         |
 | `index(arg: array, val: any) -> int`              | Index of first element in an array equal to `val`, `null` if not found                                                                    | `index(["i", "j", "k"], axis)`                              | The number, from 0-2 corresponding to the string `axis`                        |
 | `intersects(a: array, b: array) -> array \| bool` | The intersection of arrays `a` and `b`, or `false` if there are no shared values.                                                         | `intersects(dataset.modalities, ["pet", "mri"])`            | Non-empty array if either PET or MRI data is found in dataset, otherwise false |
 | `length(arg: array) -> int`                       | Number of elements in an array                                                                                                            | `length(columns.onset) > 0`                                 | True if there is at least one value in the onset column                        |
@@ -296,6 +297,7 @@ The following functions should be defined by an interpreter:
 | `substr(arg: str, start: int, end: int) -> str`   | The portion of the input string spanning from start position to end position                                                              | `substr(path, 0, length(path) - 3)`                         | `path` with the last three characters dropped                                  |
 | `type(arg: Any) -> str`                           | The name of the type, including `"array"`, `"object"`, `"null"`                                                                           | `type(datatypes)`                                           | Returns `"array"`                                                              |
 | `unique(arg: array) -> array)`                    | The unique values of the input array, retaining their input order. Equal float and int values are not considered distinct.                | `length(unique(columns.X)) == length(columns.X)`            | True if column `X` contains no duplicate values.                               |
+| `zip(a: array, b: array, ...) -> array`           | An array of arrays, where the i-th element of the result array contains the i-th element of each input array, in order.                   | `zip(columns.participant_id, columns.session_id)`           | Pairs of participant_id and session_id entries in a TSV file.                  |
 
 #### The `exists()` function
 
diff --git a/src/schema/meta/context.yaml b/src/schema/meta/context.yaml
index 7b630d61f5..80949e8ff4 100644
--- a/src/schema/meta/context.yaml
+++ b/src/schema/meta/context.yaml
@@ -39,7 +39,6 @@ properties:
       - ignored
       - datatypes
       - modalities
-      - subjects
     additionalProperties: false
     properties:
       dataset_description:
@@ -63,23 +62,20 @@ properties:
         type: array
         items:
           type: string
-      subjects:
-        description: 'Collections of subjects in dataset'
+      participants_tsv:
+        description: 'Contents of /participants.tsv, accessed by column header'
         type: object
-        required:
-          - sub_dirs
-        additionalProperties: false
-        properties:
-          sub_dirs:
-            description: 'Subjects as determined by sub-* directories'
-            type: array
-            items:
-              type: string
-          participant_id:
-            description: 'The participant_id column of participants.tsv'
-            type: array
-            items:
-              type: string
+        additionalProperties:
+          type: array
+          items:
+            type: string
+      sessions_tsv:
+        description: 'Contents of /sessions.tsv, accessed by column header'
+        type: object
+        additionalProperties:
+          type: array
+          items:
+            type: string
   subject:
     description: 'Properties and contents of the current subject'
     type: object
diff --git a/src/schema/meta/expression_tests.yaml b/src/schema/meta/expression_tests.yaml
index ff6e1ab73e..9b65a38839 100644
--- a/src/schema/meta/expression_tests.yaml
+++ b/src/schema/meta/expression_tests.yaml
@@ -26,6 +26,8 @@
   result: false
 - expression: intersects(null, [])
   result: false
+- expression: glob(null)
+  result: null
 - expression: allequal([], null)
   result: false
 - expression: allequal(null, [])
@@ -48,6 +50,14 @@
   result: null
 - expression: unique(null)
   result: null
+- expression: zip([], null)
+  result: null
+- expression: zip(null, [])
+  result: null
+- expression: zip(null, [1])
+  result: null
+- expression: zip([1], null)
+  result: null
 - expression: type(null)
   result: 'null'
 - expression: null == false
@@ -157,3 +167,7 @@
   result: [1]
 - expression: unique([1.0, 1])
   result: [1.0]
+- expression: zip([1, 2, 3], [4, 5, 6])
+  result: [[1, 4], [2, 5], [3, 6]]
+- expression: zip([1, 2, 3], [4, 5, 6], [7, 8, 9])
+  result: [[1, 4, 7], [2, 5, 8], [3, 6, 9]]
diff --git a/src/schema/objects/columns.yaml b/src/schema/objects/columns.yaml
index 1ec63d9c2d..43500e4ac0 100644
--- a/src/schema/objects/columns.yaml
+++ b/src/schema/objects/columns.yaml
@@ -533,6 +533,14 @@ response_time:
     `n/a` denotes a missed response.
   type: number
   unit: s
+run_id:
+  name: run_id
+  display_name: Run ID
+  description: |
+    A run identifier of the form `run-<index>`,
+    used to distinguish separate acquisitions with the otherwise same properties.
+  type: string
+  pattern: ^run-[0-9]+$
 sample_id:
   name: sample_id
   display_name: Sample ID
diff --git a/src/schema/objects/datatypes.yaml b/src/schema/objects/datatypes.yaml
index b3fdc455e3..65a42786e4 100644
--- a/src/schema/objects/datatypes.yaml
+++ b/src/schema/objects/datatypes.yaml
@@ -69,7 +69,7 @@ phenotype:
   value: phenotype
   display_name: Phenotype
   description: |
-    Participant level measurement data
+    Participant-level tabular measurement data
     (for example, responses from multiple questionnaires)
     split into individual files separate from `participants.tsv`.
 nirs:
diff --git a/src/schema/objects/entities.yaml b/src/schema/objects/entities.yaml
index 7a6dd88af4..c442c66bea 100644
--- a/src/schema/objects/entities.yaml
+++ b/src/schema/objects/entities.yaml
@@ -490,6 +490,13 @@ template:
     [Coordinate Systems Appendix](SPEC_ROOT/appendices/coordinate-systems.md).
   type: string
   format: label
+tool:
+  name: tool
+  display_name: Tabular measurement tool
+  description: |
+    An alphanumeric name for a phenotypic and assessment data tabular file.
+  type: string
+  format: label
 tracer:
   name: trc
   display_name: Tracer
@@ -502,16 +509,6 @@ tracer:
     Please note that the `<label>` does not need to match the actual value of the field.
   type: string
   format: label
-volume:
-  name: voi
-  display_name: Volume of Interest
-  description: |
-    The `voi-<label>` entity can be used to distinguish acquisitions localized to different regions.
-    The label SHOULD be the name of the body region or part scanned.
-    If used, the fields `"BodyPart"` and `"BodyPartDetails"` MUST be defined in the JSON file.
-    `BodyPartDetailsOntology` is OPTIONAL to also include.
-  type: string
-  format: label
 tracksys:
   name: tracksys
   display_name: Tracking System
@@ -526,3 +523,13 @@ tracksys:
     may be longer and more human readable.
   type: string
   format: label
+volume:
+  name: voi
+  display_name: Volume of Interest
+  description: |
+    The `voi-<label>` entity can be used to distinguish acquisitions localized to different regions.
+    The label SHOULD be the name of the body region or part scanned.
+    If used, the fields `"BodyPart"` and `"BodyPartDetails"` MUST be defined in the JSON file.
+    `BodyPartDetailsOntology` is OPTIONAL to also include.
+  type: string
+  format: label
diff --git a/src/schema/objects/files.yaml b/src/schema/objects/files.yaml
index 9c6bc41305..9fa03d972d 100644
--- a/src/schema/objects/files.yaml
+++ b/src/schema/objects/files.yaml
@@ -69,15 +69,19 @@ participants:
   file_type: regular
   description: |
     The purpose of this RECOMMENDED file is to describe properties of participants
-    such as age, sex, handedness, species and strain.
+    such as age, sex, handedness, species, and strain.
     If this file exists, it MUST contain the column `participant_id`,
     which MUST consist of `sub-<label>` values identifying one row for each participant,
     followed by a list of optional columns describing participants.
-    Each participant MUST be described by one and only one row.
+    For participants with multiple sessions, the `session_id` column is RECOMMENDED.
 
     The `participant_id` entries MUST be a superset of all subject directories
     and all `participant_id` entries found among phenotypic and assessment data
     in the `phenotype/` directory.
+    When in use, the `session_id` entries MUST be a superset of all session directories,
+    all tabular phenotypic `session_id` entries found among phenotypic and assessment data
+    in the `phenotype/` directory, and all `session_id` entries found in the
+    [sessions file(s)](SPEC_ROOT/modality-agnostic-files/data-summary-files.md#sessions-file).
 
     Commonly used *optional* columns in `participants.tsv` files are `age`, `sex`,
     `handedness`, `strain`, and `strain_rrid`.
diff --git a/src/schema/objects/metadata.yaml b/src/schema/objects/metadata.yaml
index 347f6a53c0..b787bc5e59 100644
--- a/src/schema/objects/metadata.yaml
+++ b/src/schema/objects/metadata.yaml
@@ -68,6 +68,18 @@ AcquisitionVoxelSize:
     type: number
     exclusiveMinimum: 0
     unit: mm
+AdditionalValidation:
+  name: AdditionalValidation
+  display_name: Additional Validation
+  description: |
+    A string or list of strings of additional validations to be performed on the data,
+    chosen from among a pre-defined set. The currently allowed values are
+    only `"Phenotype"`.
+  anyOf:
+    - type: string
+    - type: array
+      items:
+        type: string
 Anaesthesia:
   name: Anaesthesia
   display_name: Anaesthesia
@@ -1892,6 +1904,15 @@ Immersion:
     Lens immersion medium. If the file format is OME-TIFF, the value MUST be consistent
     with the `Immersion` OME metadata field.
   type: string
+IndexColumns:
+  name: IndexColumns
+  display_name: Index Columns
+  description: |
+    A list of column names to be used as joint indices in
+    their corresponding tabular phenotypic data file.
+  type: array
+  items:
+    type: string
 InfusionRadioactivity:
   name: InfusionRadioactivity
   display_name: Infusion Radioactivity
@@ -2561,6 +2582,7 @@ MeasurementToolMetadata:
     Contains two fields: `"Description"` and `"TermURL"`.
     `"Description"` is a free text description of the measurement tool.
     `"TermURL"` is a URL to an entity in an ontology corresponding to this tool.
+    RECOMMENDED by `AdditionalValidation` containing `"Phenotype"` in `dataset_description.json`.
   type: object
   properties:
     TermURL:
diff --git a/src/schema/objects/suffixes.yaml b/src/schema/objects/suffixes.yaml
index 37cca59edf..7df06a7821 100644
--- a/src/schema/objects/suffixes.yaml
+++ b/src/schema/objects/suffixes.yaml
@@ -814,6 +814,11 @@ phasediff:
     unique `phasediff` file.
     For instance, this is a common output for the built-in fieldmap sequence of
     Siemens scanners.
+phenotype:
+  value: phenotype
+  display_name: Tabular phenotypic data
+  description: |
+    A table of data in a tab-separated value (TSV) file.
 photo:
   value: photo
   display_name: Photo File
diff --git a/src/schema/rules/checks/dataset.yaml b/src/schema/rules/checks/dataset.yaml
index a1b3cd7387..3c56d9c69a 100644
--- a/src/schema/rules/checks/dataset.yaml
+++ b/src/schema/rules/checks/dataset.yaml
@@ -12,7 +12,7 @@ SubjectFolders:
     - path == '/dataset_description.json'
     - dataset.dataset_description.DatasetType != "study"
   checks:
-    - length(dataset.subjects.sub_dirs) > 0
+    - length(glob("sub-*")) > 0
 
 NoSubjectFolders:
   issue:
@@ -24,7 +24,7 @@ NoSubjectFolders:
     - path == '/dataset_description.json'
     - dataset.dataset_description.DatasetType == "study"
   checks:
-    - length(dataset.subjects.sub_dirs) == 0
+    - length(glob("sub-*")) == 0
 
 # 49
 ParticipantIDMismatch:
@@ -38,10 +38,23 @@ ParticipantIDMismatch:
     - path == '/participants.tsv'
   checks:
     - |
-      allequal(
-        sorted(intersects(columns.participant_id, dataset.subjects.sub_dirs)),
-        sorted(dataset.subjects.sub_dirs)
-      )
+      length(intersects(columns.participant_id, glob("sub-*")))
+      == length(glob("sub-*"))
+
+AgeUnits:
+  issue:
+    code: AGE_UNITS
+    message: |
+      The "Units" value for age in 'participants.json' is not a valid
+      ISO 8601-based duration unit.
+      Allowed values are "year", "month", "week", "day", "hour", "minute",
+      or "second".
+    level: warning
+  selectors:
+    - path == '/participants.tsv'
+    - '"Units" in sidecar.age'
+  checks:
+    - intersects([sidecar.age.Units], ["year", "month", "week", "day", "hour", "minute", "second"])
 
 AgeUnits:
   issue:
diff --git a/src/schema/rules/checks/general.yaml b/src/schema/rules/checks/general.yaml
index f23e81b824..adb10d2886 100644
--- a/src/schema/rules/checks/general.yaml
+++ b/src/schema/rules/checks/general.yaml
@@ -35,3 +35,50 @@ DuplicateReadmes:
     - match(path, '^/README.*')
   checks:
     - exists(["README", "README.md", "README.rst", "README.txt"], "dataset") == 1
+
+SegregatedSessions:
+  issue:
+    code: SEGREGATED_SESSIONS
+    message: |
+      Session files were detected outside the dataset root and
+      "Phenotype" additional validation has been enabled.
+      Aggregate session data in a root-level `sessions.tsv` file.
+    level: error
+  selectors:
+    - suffix == 'sessions'
+    - extension == '.tsv'
+    - intersects(dataset.dataset_description.AdditionalValidation, ["Phenotype"])
+  checks:
+    - type(entities.subject) == 'null'
+
+SessionRequired:
+  issue:
+    code: SESSION_REQUIRED
+    message: |
+      Multi-session data is detected in the dataset, and
+      "Phenotype" additional validation has been enabled.
+      Every data file MUST be found in a session directory.
+    level: error
+  selectors:
+    - type(entities.subject) != 'null'
+    - "!intersects([extension], ['.tsv', '.json'])"
+    - type(dataset.sessions_tsv) != 'null'
+    - intersects(dataset.dataset_description.AdditionalValidation, ["Phenotype"])
+  checks:
+    - type(entities.session) != 'null'
+
+SessionsTSVRequired:
+  issue:
+    code: SESSIONS_TSV_REQUIRED
+    message: |
+      Multi-session data is detected in the dataset, and
+      "Phenotype" additional validation has been enabled.
+      A sessions.tsv file MUST exist in the dataset root.
+    level: error
+    location: /sessions.tsv
+  selectors:
+    - "!intersects([extension], ['.tsv', '.json'])"
+    - type(entities.session) != 'null'
+    - intersects(dataset.dataset_description.AdditionalValidation, ["Phenotype"])
+  checks:
+    - type(dataset.sessions_tsv) != 'null'
diff --git a/src/schema/rules/checks/phenotype.yaml b/src/schema/rules/checks/phenotype.yaml
index 1e836508a5..c9b7471e43 100644
--- a/src/schema/rules/checks/phenotype.yaml
+++ b/src/schema/rules/checks/phenotype.yaml
@@ -1,6 +1,5 @@
 # Rules for phenotype files
 ---
-
 # 51
 PhenotypeSubjectsMissing:
   issue:
@@ -11,11 +10,84 @@ PhenotypeSubjectsMissing:
     level: error
   selectors:
     - datatype == 'phenotype'
-    - suffix == '.tsv'
-    - type(dataset.subjects.participant_id) != 'null'
+    - extension == '.tsv'
+    - type(dataset.participant_tsv.participant_id) != 'null'
   checks:
     - |
-      allequal(
-        sorted(intersects(columns.participant_id, dataset.subjects.participant_id)),
-        sorted(dataset.subjects.participant_id)
-      )
+      length(intersects(columns.participant_id, dataset.participant_tsv.participant_id))
+      == length(unique(columns.participant_id))
+
+PhenotypeSessionsRequired:
+  issue:
+    code: PHENOTYPE_SESSIONS_REQUIRED
+    message: |
+      Phenotype TSV files must have a session_id column if a dataset contains multiple sessions.
+    level: error
+  selectors:
+    - datatype == 'phenotype'
+    - extension == '.tsv'
+    - intersects(dataset.dataset_description.AdditionalValidation, ["Phenotype"])
+    - type(dataset.session_tsv) != 'null'
+  checks:
+    - type(columns.session_id) != 'null'
+
+PhenotypeSessionFileRequired:
+  issue:
+    code: PHENOTYPE_SESSION_FILE_REQUIRED
+    message: |
+      A phenotype TSV file contains a session_id column but no sessions.tsv file
+      was found at the dataset root. A sessions.tsv file is required to define
+      valid session labels.
+    level: error
+  selectors:
+    - datatype == 'phenotype'
+    - extension == '.tsv'
+    - intersects(dataset.dataset_description.AdditionalValidation, ["Phenotype"])
+    - type(columns.session_id) != 'null'
+  checks:
+    - exists('sessions.tsv', 'dataset')
+
+PhenotypeSessionAggregation:
+  issue:
+    code: PHENOTYPE_SESSION_AGGREGATION
+    message: |
+      Participant-level sessions files (sub-<label>_sessions.tsv) must not exist
+      when a root sessions.tsv file is present. Session information should be
+      aggregated in the root sessions.tsv file.
+    level: error
+  selectors:
+    - suffix == 'sessions'
+    - extension == '.tsv'
+    - intersects(dataset.dataset_description.AdditionalValidation, ["Phenotype"])
+    - type(entities.subject) != 'null'
+  checks:
+    - "!exists('sessions.tsv', 'dataset')"
+
+PhenotypeSessionsMissing:
+  issue:
+    code: PHENOTYPE_SESSIONS_MISSING
+    message: |
+      Session directories were found in this dataset but no sessions.tsv file
+      exists at the dataset root. A sessions.tsv file is required when session
+      directories are present and Phenotype validation is enabled.
+    level: error
+  selectors:
+    - path == '/dataset_description.json'
+    - intersects(dataset.dataset_description.AdditionalValidation, ["Phenotype"])
+    - length(glob("sub-*/ses-*")) > 0
+  checks:
+    - exists('sessions.tsv', 'dataset')
+
+PhenotypeSessionLevels:
+  issue:
+    code: PHENOTYPE_SESSION_LEVELS
+    message: |
+      The sessions.tsv sidecar must include a Levels field for session_id
+      to define valid session labels and their descriptions.
+    level: error
+  selectors:
+    - suffix == 'sessions'
+    - extension == '.tsv'
+    - intersects(dataset.dataset_description.AdditionalValidation, ["Phenotype"])
+  checks:
+    - type(sidecar.session_id.Levels) != 'null'
diff --git a/src/schema/rules/dataset_metadata.yaml b/src/schema/rules/dataset_metadata.yaml
index c206476390..322d15fc39 100644
--- a/src/schema/rules/dataset_metadata.yaml
+++ b/src/schema/rules/dataset_metadata.yaml
@@ -23,6 +23,7 @@ dataset_description:
     DatasetDOI: optional
     GeneratedBy: recommended
     SourceDatasets: recommended
+    AdditionalValidation: optional
 
 dataset_authors:
   selectors:
diff --git a/src/schema/rules/sidecars/phenotype.yaml b/src/schema/rules/sidecars/phenotype.yaml
new file mode 100644
index 0000000000..ac28665eab
--- /dev/null
+++ b/src/schema/rules/sidecars/phenotype.yaml
@@ -0,0 +1,25 @@
+---
+IndexColumns:
+  selectors:
+    - datatype == 'phenotype'
+    - extension == '.tsv'
+  fields:
+    IndexColumns: optional
+
+MeasurementToolMetadata:
+  selectors:
+    - datatype == 'phenotype'
+    - extension == '.tsv'
+    - '!intersects(dataset.dataset_description.AdditionalValidation, ["Phenotype"])'
+  fields:
+    MeasurementToolMetadata:
+      level: optional
+      level_addendum: recommended by phenotype guidelines
+
+MeasurementToolMetadataRec:
+  selectors:
+    - datatype == 'phenotype'
+    - extension == '.tsv'
+    - intersects(dataset.dataset_description.AdditionalValidation, ["Phenotype"])
+  fields:
+    MeasurementToolMetadata: recommended
diff --git a/src/schema/rules/tabular_data/modality_agnostic.yaml b/src/schema/rules/tabular_data/modality_agnostic.yaml
index 83fd7e14f4..28a1512b80 100644
--- a/src/schema/rules/tabular_data/modality_agnostic.yaml
+++ b/src/schema/rules/tabular_data/modality_agnostic.yaml
@@ -19,6 +19,28 @@ Participants:
   index_columns: [participant_id]
   additional_columns: allowed
 
+Phenotypes:
+  selectors:
+    - datatype == 'phenotype'
+    - extension == ".tsv"
+    - '!intersects(dataset.dataset_description.AdditionalValidation, ["Phenotype"])'
+  initial_columns:
+    - participant_id
+  columns:
+    participant_id:
+      level: required
+    HED: optional
+  index_columns: [participant_id]
+  additional_columns: allowed
+
+Phenotypes__Additional:
+  $ref: rules.tabular_data.modality_agnostic.Phenotypes
+  selectors:
+    - datatype == 'phenotype'
+    - suffix == 'phenotype'
+    - intersects(dataset.dataset_description.AdditionalValidation, ["Phenotype"])
+  additional_columns: allowed_if_defined
+
 Samples:
   selectors:
     - path == "/samples.tsv"
@@ -51,6 +73,7 @@ Sessions:
   selectors:
     - suffix == "sessions"
     - extension == ".tsv"
+    - '!intersects(dataset.dataset_description.AdditionalValidation, ["Phenotype"])'
   initial_columns:
     - session_id
   columns:
@@ -64,13 +87,13 @@ Sessions:
   index_columns: [session_id]
   additional_columns: allowed
 
-Phenotype:
+Sessions__Additional:
+  $ref: rules.tabular_data.modality_agnostic.Sessions
   selectors:
-    - datatype == 'phenotype'
-  initial_columns:
-    - participant_id
+    - suffix == "sessions"
+    - extension == ".tsv"
+    - intersects(dataset.dataset_description.AdditionalValidation, ["Phenotype"])
   columns:
-    participant_id: required
-    HED: optional
-  index_columns: [participant_id]
-  additional_columns: allowed
+    $ref: rules.tabular_data.modality_agnostic.Sessions.columns
+    acq_time__sessions: recommended
+  additional_columns: allowed_if_defined
diff --git a/tools/schemacode/tests/test_context_types.py b/tools/schemacode/tests/test_context_types.py
index d0692ee6d7..99f7050813 100644
--- a/tools/schemacode/tests/test_context_types.py
+++ b/tools/schemacode/tests/test_context_types.py
@@ -20,16 +20,12 @@ def test_assignability() -> None:
 
         uvx --with=. mypy tests
     """
-    subjects: p.Subjects = ctx.Subjects([])
-    subjects = ctx.Subjects([], [])
-
     dataset: p.Dataset = ctx.Dataset(
         dataset_description={},
         tree={},
         ignored=[],
         datatypes=[],
         modalities=[],
-        subjects=subjects,
     )
 
     magnitude: p.Magnitude = ctx.Magnitude("path")