The installer is only able to fetch files from one site at a time. I'm not sure if this only is valid for debootstrap, of if it's also is the case for the udebs, We use most of the udebs from upstream debian, but we also need (newer) udebs from our own mirror. http://ftp.skolelinux.no/skolelinux etch-test local

During installation, debootstrap, and later apt-install are only able to use one apt-source. We need once again to combine the two, but the goal is to keep the mirror as small as possible.

when apt-install is done, (or more correctly apt-setup) in the installer, we can add more apt-sources (and we do).