Depends what unit you get posted to I suppose. I think you can put in a preference of what unit you go to, but ultimately its where they the need people. I think you'll either go to 40,42,45 or the FPGRM to satrt with then possibly move if or when you specialize,
If you live pretty close to one of the units then its likely that is where you'll be posted. If you live in between then you normally get your first choice, but as previously mentioned it does depend on shortages etc.
I live about 30 mins drive from 40 so its unlikely they'll post me to 45 but maybe to 42 which is about 2 hours drive. I live next door to a marine in 40 and he said they are far from full capacity. Then again if all goes well its still *text deleted* be about a year before I pass RT and everything can change!
I didn't think you could be posted straight to FPGRM?