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" 
By default, light sources do not illuminate holdout objects directly.
To create a holdout light (a light source that directly illuminates other holdouts).