The X Mirror option of the Mesh Options panel allows you to edit symmetrical vertices on the other side of the mesh in a single action. When you transform an element (vertex, edge or face), if there is its exact X-mirrored counterpart (in local space), it will be transformed accordingly, through a symmetry along the local X axis.
The conditions for X Mirror to work are quite strict, which can make it difficult to use. To have an exact mirrored version of a (half) mesh, it's easier and simpler to use the Mirror Modifier.
For Topology Mirror to work the X Mirror option must be enabled.
When using the X Mirror option to work on mirrored Mesh Geometry, the vertices that are mirrored must be perfectly placed. If they are not exactly positioned in their mirror locations then X Mirror will not treat those vertices as mirrored.
Topology Mirror tries to address this problem by determining which vertices are mirrored vertices not only by using their positions but also by looking at how those vertices are related to others in the Mesh Geometry. It looks at the overall topology to determine if particular vertices will be treated as mirrored. The effect of this is that mirrored vertices can be non-symmetrical and yet still be treated as mirrored when X Mirror and Topology Mirror are both active.
The Topology Mirror functionality will work more reliably on mesh geometry which is more detailed. If you use very simple geometry, for example a Cube or UV Sphere, the Topology Mirror option will often not work.
For an example of how to use Topology Mirror open up a new Blender scene, then delete the default cube and add a Monkey object to the 3D View.
Press Tab to put the Monkey object into Edit Mode.
With the X Mirror option disabled move one of the Monkey object's vertices slightly.
Then Turn X Mirror option on again but leave Topology Mirror disabled.
If you now move that vertex again X Mirror will not work and the mirrored vertices will not be altered.
If you then enable Topology Mirror and move the same vertices again, then X Mirror should still mirror the other vertex, even though they are not perfectly positioned.
- Edge Select Mode
This select button indicates what should be done when selecting a vertex path with Ctrl-RMB:
Just selects all the edges in the path.
Marks all edges in the path as seams for UV unwrapping.
Marks all edges in the path as sharp for the Edge Split Modifier.
Marks all edges in the path as creases for the Subdivision Surface Modifier, with weight 1.0.
Gives bevel weight 1.0 (for the Bevel Modifier) to all edges in the path.
- Live Unwrap
If Live Unwrap is checked, every time an edge has its seam property changed, UV unwrap is automatically re-calculated.
- Double Threshold
Defines the maximum distance between vertices that are merged by the AutoMerge Editing tool.