It's simple mathematics. I know the exact altitude that I am ASL, so therefore, I subtract my ASL. Why use ASL...well, If I were an actual pilot in an airplane, I'd use both, MSL, as well as ASL, to keep track of how high AGL, I were actually flying. Being that land mass varies, as you fly over it. Well, same with an rc airplane, except, I don't fly CC, but instead, vertically above me, were the land isn't dramatically varying. So again, just a matter of simple mathematics. Well, hope that helps you out?! Have a pleasant day!