Create a rig using the biped proxy template. Go into one of the elbow controls and mess with the Angle Offset. Attempt to mirror the controls to the other arm. Notice that the Angle Offset does not get mirrored. It will get mirrored on the Legs, just not the Arms.
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This has been fixed in rrm3_installers_BETA_2023_06_21. However, I highly recommend against using this attribute. The rig will not behave properly in some cases, and IK/FK matching will also likely not work properly.