diff --git a/docs/data_prep.md b/docs/data_prep.md
index c2b622d..d2c31b3 100644
--- a/docs/data_prep.md
+++ b/docs/data_prep.md
@@ -5,30 +5,56 @@
     We recommend also checking out [Nipoppy](https://nipoppy.readthedocs.io/), a protocol for standardized organization and processing of clinical-neuroimaging datasets that extends [BIDS](https://bids-specification.readthedocs.io/en/stable/). 
     Neurobagel tools are designed to be compatible with data organized according to the Nipoppy specification, although you do not need to use Nipoppy in order to use Neurobagel.
 
-To use the Neurobagel annotation tool, 
+To use the Neurobagel annotation tool,
 please prepare the tabular data for your dataset as a single, tab-separated file (`.tsv`).
 
 !!! note
     In the Neurobagel context, _tabular_ or _phenotypic_ data for a dataset refers to any demographic,
-    clinical/behavioural, cognitive, or other non-imaging-derived data of participants 
+    clinical/behavioural, cognitive, or other non-imaging-derived data of participants
     which are typically stored in a tabular file format.
 
 ## General requirements for the phenotypic TSV
 
 ### All datasets
 
-A valid dataset for Neurobagel **must** include a TSV file that describes participant attributes. 
-The TSV must contain a minimum of two columns: at least one column must contain subject IDs, 
-and at least one column must describe demographic or other phenotypic information 
-(for variables currently modeled by Neurobagel, see the [data dictionary section](dictionaries.md)).
+A valid dataset for Neurobagel **MUST** include a TSV file that describes participant attributes.
+
+The TSV MUST contain:
+
+- A minimum of two columns
+- At least one column containing subject IDs
+
+    ??? note "Only one subject ID column can be annotated"
+        Neurobagel currently does not support annotating multiple subject ID columns
+        so you must choose one as the primary ID during annotation
+
+- At least one column that describes demographic or other phenotypic information
+- Unique values in the subject ID column or unique combinations of IDs if both subject and session ID columns are present
+
+The TSV MAY contain:
+
+- A column with session IDs, e.g. if the dataset is longitudinal
+  
+    ??? note "Only one session ID column can be annotated"
+        Neurobagel currently does not support annotating multiple session ID columns
+        so you must choose one as the primary ID during annotation
+
+The TSV MUST **NOT** contain:
+
+- Missing values in the columns you plan to annotate as containing the primary subject IDs and session IDs (if available)
+
+For all variables currently modeled by Neurobagel, see the [data dictionary section](dictionaries.md).
 
 ### Datasets with imaging (BIDS) data
 
 If a dataset has imaging data in [BIDS](https://bids-specification.readthedocs.io/en/stable/) format, 
 Neurobagel **additionally** requires that:
 
-- At least one column in the phenotypic TSV contains subject IDs that match the names of [BIDS subject subdirectories](https://bids-specification.readthedocs.io/en/stable/02-common-principles.html#filesystem-structure). 
-If this condition is not met, you will encounter an error when [running the Neurobagel CLI](cli.md) on your dataset to generate Neurobagel graph-ready files, indicating that your BIDS directory contains subjects not found in your phenotypic file.
+- At least one column in the phenotypic TSV contains subject IDs that 
+  match the names of [BIDS subject subdirectories](https://bids-specification.readthedocs.io/en/stable/02-common-principles.html#filesystem-structure). 
+  If this condition is not met, you will encounter an error 
+  when [running the Neurobagel CLI](cli.md) on your dataset to generate Neurobagel graph-ready files, 
+  indicating that your BIDS directory contains subjects not found in your phenotypic file.
 
     !!! note
         Subject IDs are case-sensitive and must match BIDS subject IDs exactly 
@@ -39,7 +65,10 @@ If this condition is not met, you will encounter an error when [running the Neur
 - All BIDS subjects are included in the phenotypic TSV, 
   even if they only have BIDS imaging information. 
   Neurobagel does not allow for datasets where subjects have BIDS 
-  data but are not represented in the phenotypic TSV (however, subjects who have phenotypic data but no BIDS data are allowed).
+  data but are not represented in the phenotypic TSV 
+  (however, subjects who have phenotypic data but no BIDS data are allowed).
+- If the dataset is longitudinal, the session IDs in the phenotypic TSV
+  MAY match the session IDs in the BIDS dataset, but don't have to.
 
 ## Examples of valid phenotypic TSVs
 
@@ -83,16 +112,23 @@ Example TSV:
     (see also the BIDS specification section on [Longitudinal and multi-site studies](https://bids-specification.readthedocs.io/en/stable/06-longitudinal-and-multi-site-studies.html#longitudinal-and-multi-site-studies)).
 
 ### Multiple participant or session identifier columns
-In some cases, there may be a need for more than one set of IDs 
+
+In some cases, there may be a need for more than one set of IDs
 for participants and/or sessions.
 
 For example, if a participant was first enrolled in a behavioural study
-with one type of ID, 
+with one type of ID,
 and then later joined an imaging study under a different ID.
 In this case, both types of participant IDs should be recorded in the tabular file.
 
 The only requirement is that **the combination of all ID values for a row is unique**.
 
+!!! Warning "Neurobagel currently supports only one subject ID and one session ID"
+
+    Neurobagel currently does not support annotating multiple subject or session ID columns in the same TSV file. 
+    If you have multiple subject or session IDs, you must choose one to use as the primary ID.
+    Additional subject/session ID columns can still be included in the TSV but will be ignored by Neurobagel.
+
 Example **invalid** TSV:
 
 | participant_id | alternative_participant_id | ... |