If Weights is enabled you can choose between four different methods to match points on one side of the mesh as best as possible to points on the other side.
This helps to ensure the correct transfer of the weights. Which mode works best varies from character to character. Try the other methods if the current method doesn't produce the desired results.
When matching a point, the tool selects the point that is nearest to the mirrored position within a specified distance (defined by Tolerance).
Selects the nearest point to the mirrored position without the limitations of a specified radius.
Looks for the surface closest to the mirrored position. This could be in the middle of a polygon, in which case the original point's weighting is transferred to the polygon's points.
Works in a similar fashion to Surface except that a ray is projected from the mirrored position in the direction of the point's normal. The location at which this ray hits the surface is used.
This parameter is only available if Search is set to Distance. It defines the radius from the mirrored position within which the tool searches for a matching point.