Timelapse has got it absolutely spot on.
I will add one thing which may help though. Our CDM said to us that as we were mobile grades an could be sent anywhere (you have to sign a bit of paper saying so) the best tactic regarding posting is to think of the worst possible place you could be sent and be prepared to go there. Not saying that's where you'll be but if you're ready for the worst, you can only be pleasantly surprised.
You are asked where your preference is but that is the last consideration after business need, where your skills are best suited and a hefty dose of black magic!!