This page provides information about the Material ID Number in V-Ray for SketchUp.
Material ID Number Render Element masks objects in the scene based on their shader Material ID Number value. The selection masks use automatic integer color assignment. The channel uses no anti-aliasing.
Opacity color and textures with value of 1 and 0 are supported by the Material ID Number channel.
This render channel is useful in a compositing process as a color mask for different materials in the scene.