If you’re a tank, have to queue as a tank. If you’re a healer, have to queue as a healer. Same with DPS. If you’re dual spec, can only queue as the two specs you are and automatically switch to that spec upon zone in and disallow spec changes within the dungeon.
It is so annoying to join a group as a tank and there are 3 other tanks who queued DPS because “it does better dps” and having to fight for aggro. Several runs I have been on had healers complaining about the issue as well, with some stating they’re going to stop healing tanks who join as dps.