Gtox is correct here.
Material index is to help the engine define what sounds and decals to assign to each model. Decals meaning, bullet holes, debris etc.
With regards to more texture maps, I would love the capability of adding more! Such as cavity, height, gloss. All these would be great! I think these aren't a limitation of the engine itself, more a limitation of the shaders we currently have available.
Cheers
Jay
