The pilots get to submit a limited list of preferences, but there's a fixed list of slots, like "f22 => 2, f16 => 4, ... kc-135 => 3".

Then, your ranking in the class drives when it's your turn to pick. Along with some other criteria. For example, some aircraft have min/max pilot height criteria because of the ejection seat. So some pilots do end up flying aircraft they didn't really want to fly.