I don't think there is a rule. I'm pretty sure that it's exactly as it says...2000AGL. Any graphic I've ever seen suggests it follows the terrain exactly.
However as IO540 says nobody cares. So the simpliest way to deal with it, is to ask the controlling authority for a clearance through it at an appropriate level. If it's so low that the commericals aren't interesting in using that level, then you should have no problem getting a clearance. That way you are covered, and everyone is happy.
Obviously this doesn't work so good with Class A.
dp