diff --git a/synapse/config/registration.py b/synapse/config/registration.py
index 5c2128799..34326718a 100644
--- a/synapse/config/registration.py
+++ b/synapse/config/registration.py
@@ -114,6 +114,8 @@ class RegistrationConfig(Config):
 
         # If enabled, user IDs, display names and avatar URLs will be replicated
         # to this server whenever they change.
+        # This is an experimental API currently implemented by sydent to support
+        # cross-homeserver user directories.
         # replicate_user_profiles_to: example.com
 
         # Users who register on this homeserver will automatically be joined