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The numbers are range in hundreds of metres on an M14 and yards on an M1 Garand.
You can zero the rifle at a known range (typically 200m), loosen the screw on the elevation shaft and turn the dial without moving the aperture such that the range mark lines up with the mark on the receiver. This makes it possible to quickly dial in the right elevation for a given range. The marks are of course calibrated for M80 ball or equivalent.
This target can be also used at 25m to provide a 250m zero: