Assigning Matte IDs to rendered object and outputting them as part of the AOVs can be useful for later compositing because this allows compositors to isolate objects.

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First we'll assign a Green MatteID to the sphere. We can do this by selecting the transform and adding the RenderMan user attribute for MatteID0:

If you were to render at this point, even if you added the correct render pass, it would not work. You need to connect a PxrMatteID to the material. You can do this using the Node Editor and connecting to the Utility Pattern Connection of the PxrSurface material:

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Now you can add additional PxrMatteIDs and chain together more patterns for different results in your renders. Your compositor will thank you.