Using the Mirroring section in the properties control creates mirrored reflector or lens images that are actually a property of the optic itself. So when you make changes to the optic profile, those changes will also be updated in the mirrored part of the optic. A single point and direction vector defines the mirror plane. These are given in the current CPlane coordinates.
Extruded Symmetric optics are built with this mirroring already in place. However, any parametric optic can be mirrored about an arbitrary number of planes. For example, if you are designing a forward throw roadway reflector, you will design half of the reflector and mirror it about the curb line. If you are designing a square pattern area light, then you could design 1/8 of the reflector and mirror it about three axes (a diagonal and two cardinal directions) to build the full reflector.