LAST EDIT May 03 2021

When you add a user or a collection object to an activity, Stream stores the unique reference and replaces it at read time. In some complex cases, you need to be able to generate a reference to an existing object and embed that inside of an activity.

If you are using the APIs on web / mobile (see auth section) you must set to the reference of the current user or otherwise you will get a permission error (see examples above).