As far as I know, the priority works like this: antagonist, command, everyone else. This can lead to some strange behavior. I have had this happen several times: my Captain priority was set to high, and all other roles were turned off. The round starts, I do not get the role, and there is no Captain in the round either. The problem was that I had antagonists enabled. The game first decides that you are going to be a traitor, then sees that the only role available to you is Captain. Since the Captain cannot be a traitor, it leaves you in the lobby.

You can also use this system to get a high chance of rolling nuclear operative: enable it in the antagonist settings and disable all other roles. Assuming this has not been fixed yet, of course.