Unit Models in Rome Remastered
For general modding advice see Total War: Rome Remastered - Modding
Units in Rome Remastered use the same skeleton, animations and .cas type format as the original game. Remastered allows more detailed units, better 'movement' with vertexes able to be weighted across two bones, and more sophisticated graphical effects using PBR textures.
IWTE will convert RTW models via .dae (Collada) and export in a cas format suitable for Remastered.
UV Mapping for Variations
Remastered allows variations of faces, hair and skin textures based on ethnicity. The sections of mesh that use these textures do not need to be in separate groups assigned to the textures, instead the texture used is determined by the relative position of the uv coordinates. The picture below shows a Berserker unit from Remastered with the uv mapping explained.
The textures for the face, skin and hair elements are in a long and thin format with multiple variations that the uv maps tile horizontally across to pick up the variants. The sections of texture selected for hair appear to be determined by the text file descr_unit_variation.txt. The skin and body sections presumably just scroll horizontally through their respective textures.
Unlike RTW Remastered allows full variable weighting for all 'body' components, so each vertex can be weighted proportionately across any two bones.
Primary and secondary weapons however have to each be fully weighted to just one bone. You also CAN NOT have multiple weapon components, e.g. a 'primary' shield on the back of an archer that switches to 'secondary' shield in hand.