I believe the order of template events is alphabetical, by vendor name.
Extension developers should not be setting any kind of priorities. As mentioned previously, this is impossible to control in any meaningful way, and it would be abused by some as well. It most likely will not be allowed in extensions we validate unless there is a very good reason for it.
Template event priority is the biggest concern people have raised, and it is under discussion how to address this in phpBB 3.2, because the reality is that a board admin will be the one who should control the order of events, so they can control the way their board looks.
https://area51.phpbb.com/phpBB/viewtopi ... 08&t=45227
This discussion is related. unfortunately it gets derailed into extension dependencies, which is not the point. The point is ordering template events to control the user facing experience