Siemens SINUMERIK 840DE Programming Manual page 240

Frames
6.7 Programmable mirroring (MIRROR, AMIRROR)
The mirror image refers to the coordinate axes set with G54 to G57.
Caution
The MIRROR command cancels all previously set programmable frames.
Additive instruction, AMIRROR X Y Z
A mirror image, which is to be added to an existing transformation, is programmed with
AMIRROR. The currently set or last programmed coordinate system is used as the
reference.
6-32
Programming Manual, 10.2004 Edition, 6FC5 298-7AB00-0BP1
Fundamentals