I think they are loose, held on by the wheel. The caliper holds it in place when you take the wheel off from what I remember.
Ahhh yes, I know what is being talked about now, there is a threaded hole in about the same line as the wheel studs.... yes, I would say the same as what ITS A said, for pushing the dish off if stuck.pretty sure the bolt is used to seperate the rotor from the axel if it has rusted on or stuck