PxrMatteID outputs matte AOVs for compositing.  This requires you add a User Attribute, explained in more detail here.

Add a user attribute for MatteID0.

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Set its type and color value.

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Add PxrMatteID

In your RIS PxrMaterialBuilder Network, add a PxrMatteID VOP.  Set the color as the user attribute above.

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Connect PxrMatteID's resultAOV to PxrSurface's Utility Pattern.

Set up AOV for Output

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