Reverses the normals of the selected polygon mesh. The normals define the inside and outside of the surface, which is important when using the Offset Surface method for profile lines. If the surface is offset on the inside instead of the outside you will not get correct profile lines. Also if the normals are not in the correct direction and you use the Line Offset attribute, the lines could be offset into the surface, rather than out from the surface.

In the case of NURBS or subdivision surfaces that are assigned toon lines, a conversion poly mesh would have been generated. Reverse Surfaces knows how to find and reverse this conversion polygon mesh when only the NURBS is directly selected. (The conversion meshes are drawn, but have displays set to reference object, so they can't be directly picked.) Therefore for NURBS or subdivision surfaces you must do the reverse after creating the toon line. Alternatively use Edit NURBS > Reverse Surface Direction (in the Surfaces menu set); this will affect mapped textures. This is more efficient, as otherwise the reverse normals on the conversion mesh are recomputed whenever the NURBS is modified.