True: right in the center of the length of the ski. Ex: on a 180 cm ski there would be 90 cm on either side of the midsole.
Core: it gets mounted in the center of the core, right where it is the thickest (vertically).
Sidecut center: this is where it ski is the skinniest (horizontally) under foot.
Always make sure that you're very specific when you say you want your ski mounted center.