"Here I've loaded the Unpacked OBJ Export into Blender 2.79 and the Packed OBJ Export into Blender 2.80 just to show that it doesn't matter which version of Blender you use, where you change the names of your materials is still the same.

For the Unpacked OBJ model, when imported into a 3D editor, the body part are going to be individual mesh objects, (Body, Bottoms, Eyelashes, Eyes, Hair and Top in this case). It may be more or less depending on what you added to the model when you made it in FUSE.

Each of these Mesh Objects has its own materials (and textures) and the names of these materials are seen in the Materials Panel when we select a mesh object. (In this case the materials are named Body, Bottoms, Eyelashes, Eyes, Hair and Top, the same as the mesh that uses them.)

When we export SMD/DMX files using Blender Source Tools, these material names are hard coded into the models and on compile, the model expects that the VMTs it is looking for will be named the same. So for this model I would need VMTs named Body.vmt, Bottoms.vmt, Eyelashes.vmt, Eyes.vmt, Hair.vmt and Top.vmt.

Now for the Packed OBJ import, the model was fused into one mesh object and that object only uses one material. That material is named PackedMaterial0.
Keeping things simple here, (without getting into parts that have alpha and may need their own material in Source or having to split the model up because of vertex counts), let's say I export the SMD/DMX file and compile it. The material name written in to the file is PackedMaterial0, therefore the VMT name I use must be PackedMaterial0.vmt.

If I want the VMT names to be different, I have to change the names in the Materials Panel before I export the SMD/DMX files."