Ok,,I'll give this one a shot.

Firstly, check your manual. It should have something in reference to (ATV) adjustable travel volume or (EPA) end point adjustment. Different manufactures call it a bit different but they basically mean the same thing. Actually, I think (may be mistaken) Futaba calls it ATV. Anyhow,,,what these do, is allows you to control the amount of throw on the servos. With dual rates, you could adjust the travel or deflection of the ailerons to be a lot(high rate) or be down low (low rate). This function between the rates is determined by a toggle switch,,usually in the upper right hand corner of your Tx.