The Symmetry options preserve an object’s symmetry around a plane that you specify.
When one of the Symmetry options is active, ProOptimizer looks for symmetrical pairs of edges. When it finds a symmetrical pair, it applies the same optimization on either side of the plane.
These options work best if the object is truly symmetrical; for example, if it was originally created by mirroring. If ProOptimizer cannot detect symmetry about the plane you specify, it optimizes the object as it normally does.
Points that lie on the plane itself are frozen (not optimized), because they are symmetrical by definition.
Some 3ds Max objects appear to be symmetrical but are not; for example, a surface of revolution created by the Lathe modifier. In this case, although the overall geometry is symmetrical, edges all have the same orientation in the direction of revolution, so when bisected by a plane, opposite edges face in opposite directions. In this case, ProOptimizer cannot detect symmetry, and optimizes according to the usual method.