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G20: Set Units to Inches
Units from this command onwards are in inches. Note that this is only intended to affect G0, G1 and other commands commonly found in GCode files that represent objects to print. Specifically G20 only affects: G0 to G3, G10/M568 and G92.
So you should use metric values in config.g when configuring the printer and then change to inches with G20 at the end of it if the Gcodes you want to send to move the machine are expressed in inches by default.
In RRF 2.03 and later, each GCode input channel has a separate inches/mm setting.