I did some tests with a couple of things with this unit. I did tests for Materials, Textures, and a bit with the Fx_triggers file.
Concerning the materials, I found out that it doesn't make any difference in the way the material reacts to specular whatever MaterialTag you choose in the UnitArtDefine. It's a bit of shame, and I don't know why we can choose different type of material if they all react the same, but that's what I found out.
Concerning the textures, I worked specially on the SREF file and found out that the color channels drive the intensity of the reflection (Black 000=No reflection, White 256=Full Reflection), and the Alpha channel drives the Glossiness of the Reflection, in other words, how blurry the reflection is (Black 0=Full Glossy and Blurry, White 1=Full Sharp, like Chrome).
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And concerning the DIFF file, I'm sure you all already know that the alpha channel drives the CivColor on the unit, so any part that is Black in the alpha channel of the Diff is going to be replaced (actually more multiplied) with the color of the Civ you're playing. If on your color channels, these areas are white, it's going to put the color of the Civ at 100%, and if there's already a color there, it's going to multiply it.