Any .REZ file starting with GUNS_T means that it hold texture mods. The last part of those names (ie, AR, SMG, ETC, SR) signify each type of gun (ie, AR = assault rifles, SMG = sub-machine guns, SR = sniper rifles). If you have a texture mod, you inject into the rez that has a 'T' in it, and has the correct ending (ie, if you have an m16 texture mod, you inject into GNUS_T_PV_AR, because (T) it is a texture, and (AR) it is an assault rifle.
REZ's that begin with GUNS_M art model files. That is where you inject **** b's (no space). The same naming rule applies for the ends, as it does in the texture rez's (ie, AR = assault rifle, etc..)
FX.rez contains effects, but you don't really need to know much about that yet.
The rest are more advanced, so you'll learn them as you go.