Siemens SINUMERIK 840D sl Programming Manual page 368

Coordinate transformations (frames)
12.7 Programmable mirroring (MIRROR, AMIRROR)
Deactivate mirroring
For all axes: MIRROR (without axis parameter)
All frame components of the previously programmed frame are reset.
Tool radius compensation
Note
The mirror command causes the control to automatically change the path compensation
commands (G41/G42 or G42/G41) according to the new machining direction.
The same applies to the direction of circle rotation (G2/G3 or G3/G2).
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Programming Manual, 09/2011, 6FC5398-1BP40-2BA0
Fundamentals