Here is a blurb he wrote on what they do.
*LOW* - this config is just for you IF:
-you run at ALL LOW and get 25 FPS or BELOW.
-at least 512mb-1GB or RAM.
-run this on a laptop with a DX9 GFX card.
-at least or below:
nVidia 7800 GT 256, 7900 GS 256
ATi X1800 GTO 256, HD2600 TX 256
*MED*:
-if you run at ALL LOW and get 30-50 SOLID FPS.
-equal or more than 1GB or ram
-if you have better than *LOW* requirement but less than or equal to:
nVidia 7800 GTX 512, 8600 GTS 256
ATi X1900 XT 256, 1800 XT 256
*HIGH*:
- extra extras are:terrain ambient occussion, SSAO, Edga AA, normal maps + Parralax Occussion mapping and detail textures.
-if you run at ALL LOW and can get 50+ FPS.
-if you have at least or better:
nvidia 7900 GTX, 7950 GX2, or 8800 GTS/GTX/Ultra
ATi x1950 XT, 1900/1950 XTX, HD2900 XT