Mirrors vertex positions dependant on a source geo.
Set Mirror Source: The source geometry needs to be set first, which has to be perfectly symmetrical in object X and the topology needs to be identical to the geometry you want to mirror.
Tolerance defines the threshold for the vertex symmetry match.
Mirror: Performs the mirror on any geometry which has the same topology as the source.
Direction -X is left to right / +X is right to left.
Swop Sides swops the sides.