The Requires reference provides the relationship of axes to power trains. For complex kinematics this does not need to be a one to one relationship, because more than one power train might influence the motion of one axis. This reference connects all power trains to an axis that must be actively driven when only this axis should move and all other axes should stand still.
Virtual axes that are not actively driven by a power train do not have this reference. The InverseName is IsRequiredBy.