Think you apply for the position you want,but they choose which you are more suitable for and which section of the forces you end up in.
I went for chopper pilot in army but didnt have to worry as i didnt get past first hurdle---------failed the Iq test or what ever they call it lol.Got answers right but not enough in given time.Was told by someone to just tick anything and finish,but dumb-arse didnt listen.