In the old days, you were streamed on the strength of your performance at OASC/personality type(Myers Briggs etc ie: ESTJ for ALM) and other psychological factors. This meant that on the whole, those with a flair for languages went linguist, the introverted reflectors went AEOp, the technologically advanced went Air Eng and the gobby pragmatists went ALM, the really gobby Rotary.
Now it is very much a lottery based on vacancies and the observations made by training staff at Cranwitz.
Before anyone straps on the gloves for a bit of sparring, I will qualify that by having witnessed the process from most angles.
Anyway, you have to be in it to win it.