While a Blender object can contain multiple materials, turning a Blender object into a rFObject limits the Blender object's behavior. We need to treat the rFObject as _the_ object. The rFObject contains metadata for a rFactor 2 GMT.
The Traveller scripts have two methods for creating a rFObject:
Add rFactor2 Object &
Add rFactor2 Material. Both result in a rFObject containing a single rFMesh with a single (temporary) rFMaterial. A single rFMesh can have only one rFMaterial. Although, a rFObject can have multiple rFMesh/rFMaterial entries.
To match the pattern of a GMT, each rFMesh in a rFObject can have only one rFMaterial. Each rFMesh contains its own rFMaterial metadata.
With the scripts you can create only one rFMesh at a time. To have more than one rFMesh in a rFObject, you need to build each Blender mesh (or the auto-generated rFObject cube) such that it will only have one rFMaterial. Using the scripts, each Blender mesh (or auto-generated cube) becomes a rFObject/rFMesh/rFMaterial in order to create the needed rFMesh/rFMaterial item.
Once you have each Blender mesh (or auto-generated cube) as a rFObject/rFMesh/rFMaterial, you can
Outliner:Select Hierarchy and re-parent the rFMesh/rFMaterial to another rFObject. This action will leave behind the previous rFObject and any other rFMesh/rFMaterial items that were in the previous rFObject. When there are no other rFMesh/rFMaterial items you want to keep in the previous rFObject, the previous rFObject can be deleted.
This process will provide a rFObject (GMT) that rFactor 2 will see as a single object with multiple materials.
If you build a single Blender object that you want to put multiple materials on, you will need to do a
Edit Mode:Mesh:Separate:Selection. Selection being the polys which will receive the same single material. Use the
Add rFactor2 Material menu option on each separated selection to create rFObjects. As above, select the first level child (the rFMesh/rFMaterial) of the rFObject, using the shift key, drag & drop the rFMesh/rFMaterial on the desired rFObject (the new parent).
Once you have added the rFMesh/rFMaterial to the rFObject, you can still modify the mesh, the UVs, the materials, etc. without having to extract the rFMesh/rFMaterial from the rFObject. If you do extract the rFMesh/rFMaterial, the extraced item will no longer function as a rFObject and may not retain all of the rFMesh metadata.
The simplified rFObject hierarchy looks like:
rFObject
rFMesh
rFMaterial
rFMesh
rFMaterial
rFMesh
rFMaterial
...etc...
Note: Current version of the scripts may label what I'm calling a rFMesh with a suffix of '_OBJ'. This relates to the fact that Blender calls that level 'Object Properties'. The scripts consider this level a 'MESH', thus my label of 'rFMesh'. Sorry about the confusing labeling.
Hope all that makes sense.