In those distant days I did very well in getting posting of choice ...
As Mysteron so nicely expressed it, you volunteer for something reasonable. My offer to be CAS was [wisely] rejected, but otherwise a carefuly analysis of how "they" would see your choices seemed to work for me. I got the area [more exactly, co-location with serving spouse] and generally jobs that fitted my profile.
I was, however, once told I was going to a quite unsuitable post along the lines of GrumpyGit's "bad deal". The conversation with my desk officer [as one senior officer to another] was "robust" to say the least, including the guarantee that I would PVR, fail any relevant courses and generally create more trouble than his ill-judged choice would justify. I did not go to that job.
Sadly, with MUCH smaller forces than we used to have, it must get difficult moving the various pegs around the board.