Part workbench MirrorStart a new sketch. Draw a square or rectangle on XY plane and create a pad. The rectangle should be symmetric to axes.
Make a pocket on a side face.
Open Part workbench, use the mirror (Mirroring a selected shape) tool and select a plane (YZ or XZ) in Tasks tab.
Confirm operation, now you should see second, mirrored solid. You can hide Pocket or Pocket (Mirror #1) with [Space].
Part Design workbench MirrorDelete or hide Pocket (Mirror #1) and back to Part Design workbench. Select Pocket and use the mirror (Create a mirrored feature) tool. Set the plane to YZ (or XZ). As you can see, Part Design mirror works different. The pocket feature is mirrored instead the solid object.
Version: 0.13.1489 (Git)