Modes

Grid Patch

Used to contrain control points to keep the patches symmetric. Typically used for a car hood or roof to maintain continuity when passing through the mirror plane. It acts only on the positions of existing control points and don't add new ones.

If you are looking for classic mesh symmetry, use the default mirror modifier

Currently only supported for Bezier surfaces, or NURBS with automatic knots.

Curve or FlatPatch

Mirror

Complete symmetry of the wires

Points

For local symmetry. Symmetrizes only the control points selected by the factor

Segment

Symmetrizes the selected segment