As part of CD creation, we end up copying lots of files into the new CD image. For most of those files, it's obvious where they come from (e.g. binary and source packages). For some, however, we end up copying just certain files from a package onto the CD; the most common example here is the boot loader, the software that your computer runs directly from the CD to start the installation system.
To help people find the sources for those parts of the system, this directory contains the binary packages and matching source packages that contain them.