Why are the axes of a mate connector labeled "Primary" and "Secondary" rather than Z or X?

I was in a situation where I was creating a mate connector relative to an x,y,z axis system that was part of an imported file. The import had three curves labeled as X axis, Y axis, and Z axis at the origin. So I selected the origin for the MC, then I wanted to align the Z axis of the MC with the labelled Z axis of the import, and I noticed that for some reason the naming convention for the Z axis is "Primary axis". Why not just call it the Z axis? seems like it would be less confusing.