A Volume object or set is always linked to an external .vdb file. This can initially be non-existent if the Volume was created outside of Cinema 4D (e.g., using a Volume Builder that was made editable). When the scene is saved, a still non-existent *.vdb file will be created and saved with the scene file in a folder named vol. This relative path will be set automatically.
The file path can be changed at any time to create a new link. You can create a new relative path by simply entering a new file name (which will be saved/ found in a vol folder where the scene is saved). You can define an absolute path by clicking on the button at the right of the path and selecting the new path.
If you add new Volumes to a Volume Set (or remove existing ones using the Remove option), the current state can be saved to the OpenVDB file by clicking on this button. This will modify the contents of the external file (even if it wasn’t created in Cinema 4D).
Since Volumes saved in external files can be modified, clicking on this button will update the Volumes in your scene.
Volume objects can be dragged into this list or are already there if a respective OpenVDB file was loaded. Each item in the list is followed by a check box with which the item can be enabled or disabled. Disabled Volumes will not be modified (e.g., by a Volume Filter) or displayed in the Viewport.
This function lets you place Volumes selected in the list into the Object Manager as separate objects.
Here, the number of Voxels and required memory for all active Volumes in the list are displayed.