On successful completion of training you can select a posting preference. In my case 42 Commando was the first of my 7 choices and 45 Commando was the last. Needless to say I was selected for 45.
Luckily someone in my squad had selected 45 as his first choice and 42 as his last choice and had obviously been selected for 42. We managed to swap drafts. Go figure. I doubt whether things have changed very much.
you get to put down a preference but, if bods are needed at a particular unit that is where you'll go, in my experiance about 50% went where they ask the others went wherever, dont know how they decide.