Chris, I am presumng that you're talking about Assemblies, since you use the term "mates". If you are thinking in terms of Parts then it's probably better to be talking in terms of "relationships". You can also use the term "constraint" pretty much interchangeably with either "mate" or "relationship" but that gets you into an area where your terminology could be ambiguous.
Think of the coordinate system as being comprised of the three basic pre-existing planes that all run through the origin. You can Mate your point or vertex (or edge or face or plane) as Coincident to any of those planes or at a Distance from those planes. You can also Mate an edge, face or plane at an Angle to those planes. And of course you can Mate a point, vertex, edge, face or plane to the Origin as Coincident or at a Distance to it. You can even use Sketch elements in a Sketch which is the basis for a Feature to create Mates, but first you need to right-click on the Sketch and choose "Show sketch" so you can pick one or more Sketch elements.