So, if you decide to, and can role play up the circumstances necessary, you can take your "probably not Sergeant" to the position of "Commander". I get that this is a way to get some leadership abilities, particularly if your character initially didn't take Sergeant, to start with them (not that Sergeant gets NOTHING from this Advance), but what does the acquisition mean, "in-game"? You weren't an Officer, before, so you probably won't be promoted to one now; to the best of my knowledge, most real-world militaries don't even do this, with later Officer training being a rare thing, and a step down in pay, after climbing the rungs of NCO positions (I'm not a soldier, so I don't entirely know what I'm talking about), and the feudal system of 40K seems to favor being born into Officer ranks, anyway, or at least starting as a Cadet, or Lieutenant. Is there some strange position they promote you to, from whatever rank your 'typical" Guard squad soldier, taking orders from a Sergeant, usually is? Does the Sergeant conflict with another "mini-Sergeant"? Do we just assume that the brass gave you a medal, and the perk is the Advance, as opposed to the many others you could earn, that have little static bonuses?

Sorry to be weird about this, but my own knowledge of the military hierarchy is limited, whether IRL, or 40K, and when rank matters in military, I want to know what actually happens to a character who gains this Advance, both on paper (their files,not your sheet), and among the other soldiers. I'm writing a story, and the main character is this guy; a nice, slightly naive soldier who has some skills he didn't realize, and did something extraordinary, which resulted in him getting this bonus. Then, he gets sent on a critical rescue mission, because he's apparently that good, and stuff ensues. I WAS going to say he got promoted to Lieutenant, which old fluff characters, like Bastonne, made seem possible, with grunts, but stuff I've read since seems to imply that NCO and Commissioned Officer don't cross, and so I don't know if said soldier "just" got a medal, maybe some laurels to wear, i don't know, and his squad mates, even the Sergeant, are likely to listen to him, or if he actually got some quantifiable reward, to reflect how he gained the Commander Elite Advance, despite not being a Sergeant, already having a Sergeant, and "Commander" being an Officer rank. Any help on what all this means, if you willingly read this far, would be great! Thanks, and have a great day.

I'd start off by not equating the Astra Militarum to a real world military, because they aren't.

Probably just a necessary field promotiont hat stuck, or a fluke of the Administratum. I'd assume stranger things are known to happen.

I think the good example of how field promotion works and how it's different from "real" officers can be found in Heinlein "Starship Troopers" - in the part describing Rico's experience in the Officer Candidate School .

In "Ghaunts Ghosts" first novel, soldier Corbec is promoted to Coronel Corbec.

http://es.warhammer40k.wikia.com/wiki/Colm_Corbec