It’s not terrain, it’s objects, usually if not exclusively vehicles (I’m sure one of the artists will clarify) .

For example Red Dog created a gorgeous ViperDrivers KF-16-52 high res skin. If I have it in my install (\Falcon BMS 4.33 U1\Data\Terrdata\objects\KoreaObj_HiRes folder) and have the High Resolution Textures config option ticked I will see that skin (there is no KF-16-52 high res skin in the vanilla install). If I do not have that config option ticked I will see the normal (low res) skin that came with BMS 4.33.1.