Thanks for the quick reply! (Sat night too!)

But how for example, does Mach3 differentiate between the two?

When I import Gcode, the CNC moves the tool to act upon those coordinates in the G-Code - Mach3 doesn't ask "are these MCs or WCs?") ...therfore if, back to my example, I have a countour line running along X0, the tool will try & get 3mm past that (the diameter of the tool) , but the homing switch will inhibit that.

So how does CNC app differentitate between a home MC of 0,0 & then subsequently interpret / apply WC 0,0 to a different position?

I guess I'm trying to figure how you have two set of coordinates in practise!

Does that make sense?