Exponential. This is an adjustment found on many, but not all, radios. Its hard to understand at first but it alters the ratio of movement in the input (radio) to the output (servo). An EXP of zero means if you move the steering wheel or throttle 50% the servo moves 50% of its travel. Altering the number in the negative direction alters the ratio, so if you more the throttle 50% it may only move the servo 25%. This helps on those days where you are over caffeinated.