What is the difference between [unit].nif and [unit]_fx.nif files

[Rabbit, White]

I can't figure it out. When opening them in the NIF Viewer, sometimes the regular file would have team color, while the _fx not. Other times it seems as though the _fx file shows (in bright pink for some reason) the glossmap (over the texture), and other times it's not clear at all what's going on.

Any ideas?