When you have both the 'Notifications and subscriptions Module' and the 'Discussions Module' enabled, the end-users will have the following features:

• They will be able to subscribe/unsubscribe to receive notifications when discussion messages are created in a specific record
• When an end-user creates a discussion thread or adds a message in a thread, automatically will get subscribed to receive notifications when another user adds a discussion message (or new thread) in that record
• When an end-user mentions other users of the application, the mentioned users will receive notifications automatically (and won't be able to subscribe/unsubscribe to this).

This is done because when you enable both modules when running the Design System Wizard, the following notifications are created:

These notifications are sent manually in the procedure WWP_NotifyDiscussionMessage (the code is already added).