Determines which plane is used as the mirror. ZY is the default since it is used in the front view for objects with vertical symmetry, such as a face.
If the Weld Points option is enabled, points at the mirror’s edge are welded automatically — two points become one. This joins the object smoothly, avoiding a seam along the middle.
Enter the maximum weld radius in the Tolerance box. Points that are within this distance to each other will be snapped together and welded.
Enable this option if you want to ensure that the welded points (Weld Points enabled) are placed exactly on the mirror axis. Otherwise, each welded point is placed midway between the points from which it was formed — this may or may not be on the mirror axis depending on whether the points are the same distance from the mirror axis.
This option is a type of security setting that helps prevent inadvertently moving points lying on the axis very slightly during fine-tuning and this going unnoticed.
The point will snap to the symmetry plane within the tolerance defined and cannot be moved unless this option is disabled.
This is also a security setting that prevents degenerated/co-planar polygons from being created on the symmetry axis, as is possible when using the Extrude tool, for example.
When modeling, it may be necessary - depending on what is being modeled - to place the mirrored original geometry on the opposite side of the mirror plane. This can be done by clicking on the Flip button or automatically via Automatically Flip. The latter will happen if the view’s camera crosses the symmetry plane when the view is rotated.