Add Normals is a plugin for Cinema 4D R8.5 or later that will add a Normal Tag to your polygonal mesh object, or update an existing one.
With the release of version 8.0 Cinema 4D now has a 'Normal' Tag that can be associated with polygonal mesh objects. Normals (or 'vertex normals') are basically a unit vector that is perpendicular to the face of a polygon (for face normals), or the average of all the face normals that surround a vertex (for vertex normals). Face Normals can be used to determine whether a polygon is facing the camera, or away from it, so it's used in Backface Culling, as well as simple (flat) shading models. Vertex Normals are used in lighting calculations to generate smooth-shading.
Most applications these days (including Cinema 4D) don't require that the mesh have explicit Normal data attached to it, instead, they generate that information internally at runtime. However, there are some cases where you might want the output file to have the Normal information written in it (my Riptide .obj import/export plugin can read/write Normal information - IF the tag exists before exporting, or IF the normals exist in the file before importing).
So, generally speaking, if you don't know why you'd need a Normal Tag, you probably don't - C4D doesn't need a Normal Tag on a mesh in order to render it correctly... and in fact, many tools and plugins might not update the tag correctly, causing strange shading errors, once you start slicing the mesh up (adding/removing vertices). If/When this happens, you can either:
a) delete the Normal Tag
b) use this plugin to update the Normal Tag
...so what this plugin does is create a Normal Tag (normal vector information for each vertex of each polygon) on a polygonal mesh. If the mesh already has a Normal Tag, it deletes that and attaches the new/updated one.
This plugin is is available Free of charge!
from the downloads
section, but as usual, donations are always welcome