This mandatory/formal behavior that can be expected or optional? I suggest to specify the behavior. E.g. in the case of a launch based on a Link in a CDS Hooks Card, the activityType MAY left empty. When left empty the CDS Hooks CLIENT SHALL assume the activity type corresponds to the hook that was used to retrieve the card.
Also consider adding a table that maps activityType to hook.

Existing Wording:

Note: A SMART app launched in the context of CDS Hooks should generally not need to specify an activityType or activityParameters with the ui.done message, because the EHR tracks the context in which the app was launched (e.g., order entry) and can navigate to an appropriate follow-up screen based on this context.