Axis defines along what axis the mesh will be mirrored. Merge will merge vertices between two sides if they are within Merge distance. Clipping defines whether you can move vertices of one half of the mesh in the area of another. Vertex groups is meant to mirror them if they already exist ( like, b__R_Calf__ is meant to become b__L_Calf__ ), but it does not seems to work with TS4 vertex groups because of how they are named "The vertex groups you want to mirror must be named following the usual left/right pattern (i.e. suffixed by something like “.R”, “.right”, “.L”, etc.)." Mirror Object: an Object Selector to select an object (usually an empty), which position and rotation will be used to define mirror planes (instead of using the ones from the modified object).You can animate it to animate the mirror effect. Adjust edit cage to modifier results means you will see the change in Edit mode
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