This has to do with aileron centering. I don't remember ever coming across this in instructions I've had with kits or ARF'S. It's BASIC, and something everyone should know:
When setting up (centering) your ailerons, how do you position them at "0" trim? You don't want them to produce unwanted drag. You have the centerline of the wing, looking at it from the end, (a line from the leading edge of the wing to the trailing edge): Should you try to align the aileron to that, or try to align to a line parallel to the underside of the wing, or what? Is there a rule of thumb here? Does the symmetry of the wing have a bearing? Wouldn't too much up-aileron cause drag on the wing? Just how do you find that "sweet spot"?

Jackster

they should be on the same line with wings trailing edge

With a flat bottom airfoil, align the bottom of the aileron with the bottom of the wing. With a semi or full symmetrical airfoil, align the trailing edge of the aileron with a line drawn through the center of the leading edge and center of the trailing edge of the wing.

Dr.1