The way my Shadow controllers work, is that you can work outside of
all work coordinate offset systems if you want to. G54 is the first of 6 available work offsets. G53 cancels any work coordinate offset in effect.
I am not sure if the FANUC clones work that way or are you automatically in the G54 coordinate offset by default? I need to prepare about learning FANUC style, in case I upgrade machinery one day.
So far, I am using FANUC controller clones in lathe only, so I don't get to practise much with the aspects that pertain to mills.