[imiric]

> Why not `IMPORTANT:`?

Like others mentioned, it's not as an established convention as `XXX`. And it's much shorter and quicker to type, particularly convenient when you just want to rant about something, which is the usual use case for `XXX`. ;)

At the end of the day, these labels are meaningless unless the entire team is following the same conventions. So using whatever your team agrees with using is the most important thing.