Normal Tag

A Normal tag defines the Vertex Normals (in the following simply referred to as Normals) which help calculate shading. Cinema 4D normally calculates these using the Phong tag. As soon as a Phong tag is created, the Vertex Normals will be read from the Normal tag.

The following applies, depending on which tags exist:

Only a Phong tag exists: The Phong tag will control the direction of the Vertex Normals based on the angle of the adjacent polygons. For angles below the defined threshold value (Phong Angle), the Vertex Normals will be set to a median value of the adjacent polygons, which will create the impression of a smooth surface in the Viewport and for rendering. If the angle is greater than the defined threshold value, each Vertex Normal will be set to the Polygon Normal of the adjacent polygons and a visible edge will appear when rendered.
Phong tag/Normal tag exists: If a Normal tag exists, the Vertex Normals saved within this tag will be used. The settings in the Phong tag itself will have no effect on the direction of the Vertex Normals but they will nevertheless be required for the evaluation. A Phong tag is always required for the Normal tag to function correctly.

2023

With the introduction of Cinema 4D 2023.2, Deformer tools, PLA animations, etc. can be used to affect objects with Normal tags without shading causing shading artefacting. Prerequisite: Deformers/tools/animations don't modify the point/polygon count or the polygon connections, i.g, the toplogy should not be modified.

A Normal tag primarily serves the following functionalities:

Imported CAD models

Many NURBS modelers export relatively poor-quality meshes which can be recognized by the errors in shading. Several modelers let you optionally embed "Vertex Normals" (these are Normals that exist for each point; depending on the surface, this can amount to several Normals per point). This serves exclusively to display and render the object correctly.

Note please the paragrahph above

Normals can be displayed numerically (Mode|Normal) in the Structure Manager or via the Vertex Normals option in the Viewport (at times meaned, however).