Interface signals like input and output ports, DFFs, Dlats, Z/E and cut gates, Bboxes are generally referred to as they key points. Assume you have a huge design and you want to compare RTL vs netlist. You would break the design into smaller cones and then compare. Break where?(At some cut point which will be your key point for comparison). Assuming my design had 5DFFs each DFF is my cut/key point. I compare DFF1 of golden vs DFF1 of revised and so on. But to compare I need to declare them as compare points which is done using add compare points -all. It is like moving your key points into a comparison window telling these 5 are the DFFs I need to compare. Once u do that those points are now referred to as your compare points. Support point are classified into corresponding and non-corresponding points. Corresponding points are generally mapped points that are in the fan-in cone of any diagnosis point which in out case is say DFF1. Similarly unampped points in the fanin cone of diagnosis point are referred to as non-corresponding support points. So you should know the answer to your last question by now