the shop has a jig that they put on your ski with guides for where to drill into the ski. the jig can slide up and down the ski so you tell them where you want the bindings in terms of closer to the tip or closer to the tail. the easiest way to do it is to draw a line where you want the bindings, that 100% avoids confusion with the shop tech. unless they are blind, you can avoid placement errors by drawing a line on your ski, pointing to it, and saying "that's where i want my bindings"
if you don't say anything, the tech will mount your skis at recommended. in terms of park skiing, the closer you get to true center (the ski divided in half) the better the ski performs in the park. but, as you go further up, the ski starts to feel worse skiing all mtn. so you need to think of some form of compromise between how much all mtn you do and how much park you do
so when you see -1 or -2 it usually means - however many centimeters from true center