Sorry for the moment I don't know more than that!

Vol file: The vol file is a file that contains a voxel image. It is more
similar to conventional image files like tiff and png. Within the vol files
the series of channels that make up an image are stored as arrays of
double precision numbers. In addition the vol file stores the physical
dimensions of the 3d field in inches.

Import/Export Voxel Files (.Vol)

Volume files (extension .vol) contain relevant information about the voxel configuration (ie. resolution, number of channels, etc.) and the density values attributed to each location in space. Volume files can be imported or exported to/from the stand-alone Monolith application. In addition, volume files can be used in 3rd party applications, such as the Rhino and Grasshopper™ CAD environments. The Monolith plugin for Grasshopper (which is included with the installer) provides features for importing, exporting, and modifying voxel files.

IMPORT
Referenced vol file
Use this filter to insert a reference to a vol file in the layer stack.
This creates an image layer that you can move rotate and scale but
not edit. The content of the layer is linked to the referenced vol file.

In this way you can reduce the size of models that require multiple
instances of the same voxel-image to appear in different locations.
If you use referenced files it is a good idea to save your lith file using
the “save with resources” method. This will collect all referenced file
in the same folder as the lith file and convert all references to relative
file paths.