It's all based on seniority, to some extent. The junior aircraft will most likely be the smallest aircraft in the fleet because it's the lowest paid. Everyone starts out as an FO and from there we bid either for an upgrade on the same aircraft or into the FO position on a larger aircraft. It all depends where you are on the seniority list and what position you can hold with that seniority. Some places, however, have seat lock agreements, meaning once you move to a certain position on a certain aircraft, you have to stay there for a specific amount of time (usually a year). And some places (like my place) won't let you bid for a position on another aircraft that is on the same pay scale as the aircraft you are currently on.

Confused yet?