debian_apt/dselect/desc.apt

The APT installation method encompasses most other installation methods
under the umbrella of the new Package Acquisition code. This method allows
installation from locations in the filesystem, ftp and http URLs, supports
full installation ordering and dependency checking as well as multiple 
sources. See the man pages apt-get(8) and sources.list(5)

HTTP proxies can be used by setting http_proxy="http://proxy:port/" before
running DSelect. FTP proxies require special configuration detailed in
the apt.conf(5) man page (see /usr/share/doc/apt/examples/apt.conf)