This document is supposed to give you some insight where you'll find what in the source repository tree. If you're familiar with Haiku already, you will notice that the tree layout to some degree mirrors the structure a Haiku installation has on disk.
Things that are built are usually put under "src", for example, there is a "data", and a "src/data" folder - the former for example contains scripts or settings ready to be copied to the Haiku image, while the latter for example contains the MIME types database that first have to be converted from a format that is easy to edit to what Haiku expects.
