RE: GSP Spitfire ARF build
Yes, that location is for a servo driving mechanical retracts. DO NOT CUT THE COVERING!! (cause it'll look bad... and even though it won't show when the plane is assembled, you'll know about it and it'll haunt you).
The pneumatic retracts will have a nipple on the mechanism (at the end of the cylinder on Spring Air retracts). Attach the airline tubing (which should have been supplied with your retracts if you purchased Spring Air) to the nipple and feed it through and out of the top of the wing (just like your aileron servo wires). One tube will come from each of the two retract mechanisms. Where they come out of the top of the wing, you'll need to attach "quick connectors." You can use either Robart or Spring Air quick connectors. You'll have to buy these separately. Now, in the fuselage, you'll install an additional servo (maybe even a miniservo) that will operate the valve. I can attach a picture of my set-up when I get to my home computer. It's a bit hard to explain in this format.
I'll attach some more pictures of my Spring Air installation when I get home tonight.
Until then,
Noah