G20 and G21 codes change how values in a job/macro file are interpreted. If you use G20, the software will use inches for coordinates. If you use G21, it will use millimeters.

Please note that this will make your work offsets invalid. For that reason, we strongly recommend refraining from changing the coordinate system through these G-Codes or the setup interface. It is best to pick a system before setting up your machine and then keeping it.