This is two sets of replacement decals. They replace the national emblems on the tanks with either a high definition set, or a set of historical WWII era national and battle flags.
To install copy the set of files from the archive into res_mods/x.x.x/gui/maps/vehicles/decals if this folder doesn’t exist, create it. Then copy the files from EITHER decals_hd OR historic_flags into that folder. NOT the two sub directories, only the files from the folder of your choice.
For HD, unarchive the files in decals/decals_hd