This The Normals channel outputs the orientation of every surface in the image through representing the orientation of its Normal vector at this point.
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This information is useful in compositing when you need to perform adjustments dependent on the World orientation of a surface (to mimic dirt, dust, illumination, shadows, etc).
The render (left) and the Normals channel in World coordinates (right)
Normals channel in Camera coordinates
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This information is useful in compositing when you need to perform adjustments dependent on the Camera orientation of a surface (lens effects, etc).
The render (left) and the Normals channel in Camera coordinates (right)