Quake flashes the screen based on things that happen in the game (picking up ammo, getting damaged, etc.).
These are called 'cshifts', and they add to the immersion of the game, but some people prefer to turn them off. Still, some mods have used cshifts for game effects (example: some versions of Team Fortress have a "flash-bang" grenade that is supposed to blind people, and use cshifts to achieve its effect). It's important to allow people to control their preferences, but it's also important to not enable what mods call cheating, so QuakeForge implements these fine-grained controls.
All of these cvars work for both GL and software, and all of them check the value of the "cshifts" serverinfo key. cshifts is a bit field, with the following bits defined:
An admin can force any of these to be respected by choosing the numbers of the cshifts to be enforced, and adding them up.
serverinfo cshifts 15
turns them all on, while
serverinfo cshifts 10
turns on powerup and contents shifts.