This tool can be applied to polygonal SDS objects, which have the Phong shading option set. To use it, set the target object to the edit mode, select some faces/edges/points and apply the tool. Clicking the tool icon toggles between smooth and sharp shading normals. From the side menu, the desired alternative can be chosen directly as follows:
Sets the phong shading normal of every face passing trough a selected point to the common average normal. Phong shading interpolates smoothly over the selected points.
Sets the phong shading normal of every face passing trough a selected point to the geometric normal of the polygon corner in question (computed from the two surrounding polygon edges). Phong shading does not interpolate the normal smoothly but uses a private normal per face per selected point.