In movies and commercials, we often need to composite the CG objects over a live action plate. To convince the audience that the CG objects are real and belong to the scene, we need to use a holdout object to collect the CG objects' shadow and reflection that match very closely to the shadow and reflection in the live action plate.
In RenderMan, holdout objects are specified by enabling the trace attribute holdout.
Attribute "trace" "int holdout" 
Holdouts are an integrator feature implemented at the integrator level. Available integrator that support holdouts are: PxrPathTracer, PxrVCM, and PxrUPBP.
Because holdouts are considered an intrinsic geometric property when calculating illumination, LPE workflow remain unchanged for holdout geometry.
By default, light sources do not illuminate holdout objects directly. This prevents double contribution between 3d lights used to illuminate non-holdout geometry and the illumination already captured on the plate image.
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.
Light "PxrSphereLight" "spherelight" "string lightGroup" ["light__illumholdout"]
A holdout shadow pass can be output using the PxrShadowFilter plug-in. 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.
Because holdouts workflow involves multiple AOVs, rendering with adapt all enabled can reduce noise.