A .REZ mod is a modification of the texture/sound/(others) file inside a .REZ file,usually made for aesthetic or functional (to get so an advantage) purposes. This could be used in Ghost Mode to hear better footsteps, ghost breath, slashing and stabbing, C4 setting, etc. or muting annoying sounds like shoots, grenades, radio, etc. Actually, .REZ mods are detected, so, you can't use them.