diff --git a/google/cloud/dialogflow/v2beta1/document.proto b/google/cloud/dialogflow/v2beta1/document.proto
index 6b5f8f6d..94d7055a 100644
--- a/google/cloud/dialogflow/v2beta1/document.proto
+++ b/google/cloud/dialogflow/v2beta1/document.proto
@@ -133,6 +133,9 @@ service Documents {
   // content. The previously loaded content of the document will be deleted.
   // Note: Even when the content of the document has not changed, there still
   // may be side effects because of internal implementation changes.
+  // Note: If the document source is Google Cloud Storage URI, its metadata will
+  // be replaced with the custom metadata from Google Cloud Storage if the
+  // `import_gcs_custom_metadata` field is set to true in the request.
   //
   // Note: The `projects.agent.knowledgeBases.documents` resource is deprecated;
   // only use `projects.knowledgeBases.documents`.