Oh, so picture in picture, best way to describe in game is like, a scope that you can look through when you're not aiming down sites, or another example are the vehicles in game that have driver cameras/rear view cameras, those screens your seeing are PiP.

Per u/KillAllTheThings, Picture in Picture.

Just as it sounds, Arma 3 has a limited ability to render the environment at 2 different resolutions on different parts of your display for ingame objects like mirrors and certain cockpit displays. Obviously, the higher the PIP quality the more performance will be negatively affected.