Volume Set
The Volume Set is a combined object that can contain multiple Volume objects. If you load a *.vdb file, for example, that contains three OpenVDB Volumes, you can determine if these 3 Volumes should be imported as a Volume Set or as individual Volume objects (Group Objects option).
Grouping offers the advantage that the three Volumes stay congruent in a *.vdb file when you move the Volume Set.
A Volume Set can be separated at any time by clicking on the c key, which will make each on a Child object of a Null object.
Volume objects can also be dragged manually into the Volume list.
A Volume Set object can be made a Child object of a Volume Mesher and subsequently be affected by Volume Filters.