If the surface you choose as the contact surface on a cam is part of a cylinder or similar curved shape then the direct on the line of contact is clear - i.e. parallel to the "axis" of the curve. In this case you can have Automatic for Depth Display.
If the cam or follower surface is a plane then there is no implication of direction so you have to provide this by picking a couple of reference points being the ends of the line across which contact is to be made.
Hopefully attached will get through - it is the cam connection from the arm of a ball valve and the flat face of the valve plunger. The blue ring shows the Front ref point.