...

By default, light sources are considered holdouts are and do not illuminate other 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.

...

Holdout contribution can be composited over a film plate using an alpha and shadow pass as masks. The shadow pass is generated using the PxrShadowFilter PxrShadowDisplayFilter plug-in and can be optionally written to its own AOV or added to the alpha. The occluded and unoccluded input AOVs should be rendered using the holdouts LPE prefix so that they only contain the shadowing term for holdout geometry.

...

Compositing can be interactively previewed in RenderMan by using the PxrImagePlaneFilter PxrImageDisplayFilter plug-in. This will composite the film plate with the beauty channel, using the alpha channel and holdout shadow AOV to mask the plate.

...