If you’re looking for info on CustomSettings.ini, you’re most likely to find questions answered with “try this script”. You type it in, and if it works, great; if not, keep looking. It’s well-nigh impossible to find actual documentation, and the programming-by-INI-sections design is not exactly intuitive.
It turns out there’s some in the help docs for the MDT, but those are a .CHM file and Microsoft apparently doesn’t post those online.
However, some helpful (Russian?) souls have done so. Behold: Microsoft® Deployment Toolkit 2012 Toolkit Reference.