Some of the settings can be changed to affect the way things are rendered. Others are set internally by the engine and should not be changed.

For example you can change PR_Settings.HardwareMipMaps to control mipmapping, but don''t change PR_Settings.MaxLights to a larger value because it''s a maximum determined by the video card.

If you haven''t seen any of the examples change a particular setting then chances are you shouldn''t be touching it.