aa is antialiasing, af is anisotropic filtering
and when you say doom, you mean the original or 3?
af and aa are software adjusted, just older games dont have as high levels of them, so id imagine the original doom (which i never played, before my time
) wouldn't have 2x,4x and the like.
this is af according to wikipedia:
In 3D computer graphics, anisotropic filtering is a method of enhancing the image quality of textures on surfaces that are far away and steeply angled with respect to the camera. Like bilinear and trilinear filtering it eliminates aliasing effects, but introduces less blur in the process and thus preserves more detail. Anisotropic filtering is computationally relatively expensive and has only recently become a standard feature of consumer-level graphics cards.