This means that if you build steps next to each other, the sides will not have any texture seams, no matter what size the brushes are. If necessary, the texture plane on each brush face can be offset in multiples of 16 pixels, and the texture axis can be negated or interchanged. Levels cary their own textures with them. This bulks the file up, but it makes introducing new graphics much simpler. The bsp utility extracts the textures that are actually used from that, and puts them in the output .bsp file.
Quake never references the original wadfiles.
There is a small MAP file example . Note that this one does not describe the current MAP file format, as two floating point numbers have been added to each Plane definition. The Quake qtest1 release data structures are described in the current version of the Unofficial Quake Specs . Single threaded, but most levels only take a minute or so to run. Fully parallelel, generally taking a few minutes to make a high quality run, or less for a non-filtered version.