Spindle: refers to the shaft that receives power from the engine or motor and transmits it to other parts. Principal axis, also called "optical axis", is the abbreviation of "main optical axis": the diameter with symmetry in optical concentration. For example, the principal axis of a spherical mirror is a straight line perpendicular to the mirror through the center of the mirror. The principal axis of the lens or optical axis optical group is the connecting line of the center of each lens surface.

So you can define any axis of X, Y, Z, Y, Z as the main axis in the reference system that drives the workpiece or machining tool to rotate.