Somebody claiming to be Snit Guckfung via Standards wrote:
A message sent by the room directly to the used, probably and "headline". With a link to the room and message ID. I don't know about XEP-0224, I've not implemented it so far and Philipp Hörist explicitly asked to not use it in a previous message in this thread. XEP-0372 could do the job, but it was discussed yesterday at the council meeting that maybe your specification and XEP-0372 could be merged somehow?
I'm on the fence regarding XEP-0224 as well. It was suggested by someone else in the thread for notifications, but then it was also argued against. Its also been brought up to be used for room mentions, which is an entirely different use-case. Sounds like no one knows what XEP-0224 is supposed to be in the context of MUCs, which is worrying...
XEP-0224 is pretty clear that the purpose is to send a message which cases more of an attention-grabbing alert than the same message would otherwise have received, and that how to get the user's attention is up to the implementation. So if a client gets a message with this tag it should alert the user in some way. In a MUC usually a message does not notify the user at all, so any notification at all would be more than usual. That is why it is appropriate for "room mentions", since those don't mention anything but rather indicate that a messge should alert users.