exponential technically is a setting you can program to make your inputs less sensitive near center. It pretty much slows the rate of the servo to a determined point then after it quickly moves as it reaches the end of the throw. So if you have really large throws, the plane will be very very twitchy to fly and by adding expo you will soften it up. It kinda hard to explain without a graph. But you can see what happens on the ground when you start adding it. With a futaba 6ex the exponential setting will be a negative number. Usually sport planes you could prob use -30% through -50% on high rates and with 3d planes you could easily max the expo out as you can achieve really insane throws.

I hoped this helped, or did I totally confuse you?