When a participant is subscribed to a forum it means they will receive forum post notifications. There are 4 subscription mode options:

* Optional subscription - Participants can choose whether to be subscribed
* Forced subscription - Everyone is subscribed and cannot unsubscribe
* Auto subscription - Everyone is subscribed initially but can choose to unsubscribe at any time
* Subscription disabled - Subscriptions are not allowed

Note: Any subscription mode changes will only affect users who enrol in the course in the future, and not existing users.