I've just started the game and was wondering what the 'Fog' setting controls in this game? I noticed there's no reference to it in the manual. My initial assumption was that it controls the level of detail for a fog / mist effect. Then I started wondering if it actually controls an effect called fogging which you to be used in games some time ago to improve frame rates by creating placing distant objects in a general fog. Current.y I have it set to max and my gaming rig is well above the required specs. So I'm just wondering if this is what I should have it set to?

Yes, that is exactly what it does. Since your system is so much more powerful than the game requires there is no reason to set it below the maximum. Note that some areas have their own fog distance, which will override the global setting.