The mark on the ski represents the highest point on the skis camber which should theoretically also be the point about which the skis will turn if left to their own. That is it. You "theorectically" want your COM above this point when in your "neutral" ski stance. Problem is, we are all built slightly different (women tend to carry more weight in their hips then men, men carry more weight in their chest and shoulders) and all have slightly different boot set ups (ramp angles vary/ forward lean)), as a result our COM does vary front to back by a few centimeters from person to person....hence why some people like things a little more forward, others more back....but ultimatley they are likley adjusting their bindings down below...to get their COM above where it shoud be...over that mark.
As others have said, on a railflex binding, no reason not to play with it. If you are "hippy" go forward, slim hips and average build, likely best on the mark, lots of ramp angle and forward lean on the boots, go back.....just put your boots on, and stand sideways looking at a mirror, estimate where you COM is, and put that over the ski mark, bindings might go ahead, stay netural or in some cases go back.