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First, add the volumes to the CEL group of PrmanObjectStatements.  The volume locations can be added directly to the CEL, added via a custom expression, or set on an ancestor location (like /root) and inherited. 

Next, specify the aggregate name with the prmanStatements.primAttributes.volume.aggregate attribute.  To begin with, give all volumes in the scene the same aggregate name.  Volumes without an aggregate name will render, but will not be part of the aggregate structure.

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