From BlenderWiki
Mesh Options panel
This panel is available in edit mode, when the object being edited is of type mesh.
X Mirror ticked will activate mirror editing by default: transforms made on vertices will affect symmetrical vertices on the other side. Symmetry is considered along local X axis. If proportional editing is enabled, mirror editing is always disabled.
Topology Mirror can be ticked only when X Mirror is also ticked. In this case, also mirror editing will be on; however, matching pairs don't depend on their position along a symmetry plane, but on their position inside the topology configuration. When there is more than one match (like the case of a cube or UV sphere), no matching pair is considered.
The Edge Select Mode select button indicates what should be done when selecting a vertex path with CtrlRMB 
:
- Select just selects all the edges in the path.
- Seam marks all edges in the path as seams for UV unwrapping.
- Sharp marks all edges in the path as sharp for the edge split modifier.
- Crease marks all edges in the path as creases for the subdivision surface modifier, with weight 1.0.
- Bevel gives bevel weight 1.0 (for the bevel modifier) to all edges in the path.
If Live Unwrap is ticked, every time an edge has its seam property changed, UV unwrap is automatically re-calculated.
Double Threshold defines the maximum distance between vertices so that they can be merged in 3D View » Mesh » Vertices » Remove Doubles
