i asked this question about a month ago....and smoke 21 was very helpful and curtious and helped me out....here it is:
ok Ill break it down I have this radio and the Instructions are crap for telling you what it does
this comes from BuddaTech...)In Aritronics speak, ARC is an exponential adjustment. It stands for Adjustable Rate Control. A negative value will result in a less than linear control of the function. In other words...the servo (or ESC) will respond less qiuckly than either a linear (direct relationship) or positive setting.
For a nitro car you could dial in some negative to make the initial thorotle response less imediate. This may be useful if there is limited traction or the thorotle responds too quickly. The same can be done to the steering.
Dual Rate: Dual rate allows you to change the radius of your turn. In techical tracks you might want a high setting for sharp turns and on wide tracks you can use less steering to prevent oversteer
Sub Trim: centers the servo basicly it moves the range of movement left or right it is hard to explain (what I do for this is I accally set it to 0 then install the servo and then install the horn at center and make adjustments from there.)
EPA: END POINT ADJUSTMENT this allows you to change where the servo motion stops especially helpful for nitro engines as you don't want it to over extend the throttle arm
hopefully this will help you as it did me...