To access these options, select Mesh > Make Symmetrical from the main menu bar.
Use these options to create a new topologically symmetrical model from an existing mesh, mirroring one side of the mesh to the other over one, two, or all three axes at once. See also Apply symmetry to an existing mesh.
The first three options let you specify which axis you want to mirror across, and in which direction to copy.
Specify whether to copy the left half of the mesh to the right (Mirror Left to Right), or right to left (Mirror Right to Left). (Set to Off if you don't require symmetry across the X axis.)
Specify whether to copy the bottom half of the mesh to the top (Mirror Bottom to Top), or top to bottom (Mirror Top to Bottom). (Set to Off if you don't require symmetry across the Y axis.)
Specify whether to copy the back of the mesh to the front (Mirror Back to Front), or front to back (Mirror Front to Back). (Set to Off if you don't require symmetry across the Z axis.)
When on, Mudbox copies across the mesh based on the model's local X, Y, and Z axes.
When you create symmetry across multiple axes at the same time, use this setting to indicate which axis to use as the topological axis for the resulting mesh.
For example, if you mirror across both the X and Y axes, specify whether to use X or Y as the center of the topologically symmetrical model.
This is important later if you want to sculpt or paint the model using tangent symmetry. The topological axis determines which axis the sculpt or paint brush will mirror across when its Mirror property is set to Tangent. (See also Sculpt using tool symmetry.)