Odd question for former XI players?

[Eloah]

So I never played XI, this is my first MMO, so I have no idea how XI worked. Ive heard a few things, like how it was a drop down menu action system; which is whynthe cool downs were so long, maybe. People can explain that if they want to but what I am really curious about is the role system. In XIV we have 3 roles Tank, DPS, and Healer, and there is no "switching" between them; ie a tank can't queue as a DPS. And from what I've heard XI had a similar system. But here is what I'm not understanding. I've seen several things that indicate roles could be switched, like Ninjas or dances tanking, summoners and red mages healing, warriors and palidins dpsing. Som can someone explain how XI's system worked? Could you choose which you you wanted to play as? Or was the game more lenient in terms of party makeup? Any info would be helpfull thank you.