diff --git a/packages/ref/src/ref/datasets/base.py b/packages/ref/src/ref/datasets/base.py
index 1551d06..2995552 100644
--- a/packages/ref/src/ref/datasets/base.py
+++ b/packages/ref/src/ref/datasets/base.py
@@ -120,6 +120,9 @@ def load_catalog(
         """
         Load the data catalog from the database
 
+        The index of the data catalog is the primary key of the dataset.
+        This should be maintained during any processing.
+
         Returns
         -------
         :
diff --git a/packages/ref/src/ref/datasets/cmip6.py b/packages/ref/src/ref/datasets/cmip6.py
index b1b9561..433639e 100644
--- a/packages/ref/src/ref/datasets/cmip6.py
+++ b/packages/ref/src/ref/datasets/cmip6.py
@@ -254,6 +254,9 @@ def load_catalog(
         Iterating over different datasets within the data catalog can be done using a `groupby`
         operation for the `instance_id` column.
 
+        The index of the data catalog is the primary key of the dataset.
+        This should be maintained during any processing.
+
         Returns
         -------
         :