I think a couple of answers:
you cannot request any more of anything than your total expected costs, which is tuition plus everything else a LS thinks we need. So, in theory, you can request the full amount of stafford, 18500, even if your tuition is only, say, 5,000. However, if tuition plus room/board etc only comes to 15,000, then you cannot request a full stafford of 18,500, since the LS did not certify your budget at that level...otherwise, you can borrow up to the full amount on your financial aid summary...
stafford, as noted, can thus be used for anything...its given to school, they subtract what you owe them, and give the rest to you