The usual workflow for aggregate volumes is to create a named aggregate volume by attaching the
Attribute "volume" "string volumeaggregateaggregate"
["name"] to one or more RiVolumes primitives, where
"name" is the desired name of the aggregate volume. These volume primitives should use the PxrVolume shader to control their volumetric appearance. Once several volume primitives have been tagged with this attribute with the same name, the named aggregate volume can be supplied as the
"volumeAggregate" parameter to the PxrPathTracer integrator. This signifies that the global volume aggregate for the scene is the named aggregate volume. The named volume aggregate will now appear in renders. Control over how the volume is integrated and sampled is via parameters on the PxrPathTracer integrator.