From Quake Wiki
Thanks to a cooperative effort between QuakeSrc, LordHavoc and Tomaz a variety of new engines support a Extension-Detection system called, for the purposes of this wiki, "QSG Extensions". These are meant to be used by QuakeC modders to expand and make use of the new engine features without breaking compatability with other engines (Prydon Gate is a good example of how this can be done).
For more information on how to use these, read about the checkextension builtin.
QSG Extensions are a powerful method. Mods addapt to higher and lower profiles, and engine coders dont need to stop to add new features, and QuakeC modders dont need to stop using it. But doing in the right way, everybody can run mods.
- DarkPlaces and TomazQuake support most of the old QSG keys. Actually this keys have the widen support betwen engines.
- Telejano adds a lot of new QSG keys, but most are undocumented.
- The new DarkPlaces builds add some new and very interesting keys.
- FTE QuakeWorld adds a whole new spon on things. Visit [How FTE is broken] for more information.
- You can deactivate normal features with QSG keys. Example: If you create a engine that not support centerprints. You can use QSG_NO_CENTERPRINT. If you mod its coded checking that centerprint, will run on your engine with no-centerprint and on engines with centerprint. Negative QSG keys are usefull to use Quake1 mods with other engines that can load progs.dat files, like FungusQC.
- Builtin ranges