The best thing to do is open the shader file in Notepad, or a text reader. Most of them have a description of which textures they use and some other usefull note.
If not at the top. Scroll down the file to you see the texture section which also shows you which textures are required.
Segments use _d2, _n, _s. Except bond1's norm_spec_tangentbasis.fx it uses D,n,i instead.
Where entities can be all of them depending on which shader you use.
What I usually do is just make the 5 texturs for everything I make or get for free, so they are always shader ready.
To answer your question, the _i file can be used as a glow type texture or can add the bright speculars in characters. A few other uses depending on which shader you use. If you play with them a little then you will see which one work how with the shaders you choose to use.
Let me add to your confusion, If you rename the _i file in any weapon with a scope, to a _r file. then you get reflection in the scope glass. The _r being reflection.