Actually, I think it's name is actually Overflow tank or something cause it's really not a resivuar. Think about how your power steering/MC/Clutch cylinder resivuars work. They hold the fluid in them for recirculation or for a place to store the fluid when it's not being used. You actually check the levels on these resivuars.

The coolant overflow is different in that if it's empty, it really doesn't matter as long as the radiator is full.

Make sense?