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Attribute "trace" "int holdout" 
Holdouts are an integrator feature implemented at the integrator level. Available integrator that support holdouts are: PxrPathTracer, PxrVCM, and PxrUPBP.
By default, light sources do not illuminate holdout objects directly.
To create a holdout light (a light source that directly illuminates other holdouts), create a light using "__illumholdout" as the light group suffix. This tells integrators to consider this light as a new light source when illuminating holdout geometry.
Working with AOVs
A holdout shadow pass can be output using the PxrShadowFilter plug-in. The occluded and unoccluded input AOVs should be rendered using the holdouts LPE prefix.
DisplayChannel "color occluded" "string source" ["color lpe:holdouts;C[DS]+[LO]"] DisplayChannel "color unoccluded" "string source" ["color lpe:holdouts;unoccluded;C[DS]+[LO]"]
Compositing can be previewed in RenderMan by using PxrImagePlaneFilter SampleFilter plug-in. This will composite the beauty render over the specified image plane, using the alpha channel and holdout shadow AOV to mask the plate.