This came out of discussions in MDL-19020:

As a teacher
In order to notify students about urgent information
I should be able flag a forum message as imporant and be sent to the student urgently (i.e. mobile notification)

The message API should support:

a) prioritisation of messages
b) A digest output plugin, which replaces the forum digest, collects messages and then sends them as a digest

Then the forum types could send a message and some messages could be delivered instantly by mobile notification and some could be queued into a digest to be sent out daily. Replacing the behaviour in mod_forum