Hey Don,
G54 – G59.3 (Select Coordinate System) are commands that make an offset active. As a result of such a command, you get the zero coordinate set to somewhere else than it was before. After the command, you run a g-code that mills something. It will take as the zero coordinate where the offset you activated before told it to. This way you can clamp up to 6 (9 on buildbotics) similar workpieces on different positions on the wasteboard and run the same milling sequence on them one by one, either manually, or automatically with one single program that switches through the G55-G59.3 offsets and calls the milling program as a subroutine.