Promotions are given based on activity within official RPG scenarios, although other scenarios and general activity on the message boards is also taken into account. You can receive your first promotion after just a couple of good posts in an RPG!
At higher ranks, more assignments are open to you. For example, once promoted to Sergeant, a soldier can command an entire squad of troops. You'll be moved up the chain as is appropriate for your rank.
Many members think that if there's an NPC above them, they won't be able to progress. This isn't true! NPCs can be re-assigned, transferred away from Rogue Squadron or even killed—they'll never stand in the way of your advancement.
Likewise, just because there's an empty position above you doesn't mean you automatically get promoted, you have to earn it!
You can request a change of assignment at any time through your home page, but be warned that if you change around too many times, you'll end up being refused transfers. Assignments can't be changed midway through an RPG, unless there is an in-scenario reason, so any submissions during an RPG will be deferred till its end.
You'll only ever be given an assignment befitting your rank!