I think it's a negative expo, but I'm not quite sure. But what you are trying to accomplish is less sensitive around the center of the sticks. Almost everybody uses it, especially when you get planes with large control surfaces. I usually use about 30% to start. I use it on all control surfaces. Generally what this does is when you move the stick 1/2 way the control surface doesn't move the same, it moves less, maybe only 25%. But when you move the stick 100% you move the control surface 100%. So you are less sensitive on the 1st part of the stroke and more on the latter part. You should also use dual rates for the same effect and use expo on both sides of that as well. Makes a real difference. Especially in windy conditions.
Thanks,
Barry