These settings allow you to define a symmetrical plane. Polygon Cruncher will optimize taking care of this plane. If symmetry is partial, only parts that are symmetrical will be affected by these settings. Others parts will be optimized normally.
Polygon Cruncher will look for symmetrical pairs of edges. When a point is modified during optimization, its symmetrical is optimized in the same way too.

Notes:

• Points that are on the symmetrical plane are frozen, because they are there self-symmetrical.
• Beware, to be symmetrical an objet should be mirrored constructed from both parts of the symmetrical plane. Some revolutions objects (such sphere), are not symmetrical because faces edges are oriented in the same direction on each side of the symmetrical plane.

By default these settings are checked. They give better optimization results in most of case.
If you want to apply a really high optimization ratio it can be interesting to try to uncheck these settings as they add constraint to optimization and modify the natural optimization order for removing faces and points.

A face is compact when it is not sharp. It means that it does not have any sharp edge and the triangle it made is nearly equilateral.
When this option is checked, Polygon Cruncher verify that the removal of one face does not create sharpens faces. This gives a better repartition of faces among the object. Faces are better equilibrated.
You may get better result with this setting unchecked if the original object as a lot of sharpens faces.

Prevent normals inversions

When optimizing Polygon Cruncher moves points. It can occur that these moves cause normals inversions. Resulting object seems to have holes.
Check this setting to prevent normals inversions.