So, here's the skinny:
The first "Anti Aliasing" slider controls the number of samples (2x, 4x, 8x, 16x,). Selecting a setting of "0" enables the second slider, called "Non-Maskable Anti-Aliasing (Advanced)". This is for users who have driver bugs and cannot use the first slider without issues/artifacts. This second slider ranges from 0 to 100%, and it does pretty much what the first one does, but using a different method. Additionally, users may theoretically (depending on the video driver) override the in-game AA settings with their control panel AA settings when both sliders are set to 0.
So, the official stance on this is:
- To set AA via Steal Beasts, use the in-game sliders, and set the video driver control panel setting to "Application controlled."
- To set AA via the video drivers, set both in-game sliders to 0, and use the driver control panel AA settings. (not guaranteed to work, though.)
- Anything else can, and often does, produce "undefined" behavior...