Every .REZ has tons of stuff in it , so you can't really know what each .REZ does.
You can try that by using CFREZ.exe (If you knew how to use it) and explore what each .REZ beholds , some has characters, weapons , boxes ......
What you can do with .REZ you can edit them using different programs and make hacks just as (Speed , No reload , Wallhack) It's patched but I have found a way around it , when i find another way ill share with you guys one of them.