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Metamorphic Normal mode gives you the tools to influence the normal map for an object. First you need a Normal map to apply edits to and you can choose a pre-existing Normal Map or create a new one in the Normal Map drop-down. Once done, make sure you choose the map in the Surface Editor's Vertex Map drop-down.
There are several tools:
Comb - The default tool, allows you to comb normals in screenspace to change their angle
Recalc - Recalc takes account of any deformation to your Metamorphic object, whereas Restore will simply put the normals back to their undeformed state
Restore - Brush-based undo, paint to take normals back to their inital state
Bake Normals - Only works with polygonal meshes (rather than subpatch) and removes the need for a Vertex Normal Map