RE: GP viper and 46FX
Rich,
On Futaba radios, negative expo makes the stick "LESS" sensitive around center and this smooths out the flying of a plane that might be too sensitive to you. Positive expo would make the plane even more sensitive around the center position. Most of the time you would use negative expo on Futaba. Expo is a personal choice. Some people love it and some don't. I didn't use expo for the first five years I was flying and then I had a plane that was very pitch sensitive. Someone took my radio and programmed in some expo on the elevator and it flew much smoother for me.
JR is opposite of Futaba. I'm not sure about the other manufacturers.
Marty