Debian's version of Sysv init uses scripts which reference various LSB "virtual" boot facilities such as
$network as requirements for use by insserv in setting up the boot sequence, but they are not necessarily provided by any
/etc/insserv.conf refers to them to, but only to define what services must "declare the required dependencies of the facility". To me, this would mean that the list of services "must declare the dependencies of the facility", but perhaps it is just a poor choice of words, since looking at an actual
insserv.conf implies that it actually means this list "must declare the facility as a dependency", which is very different (any clarification of that would be appreciated).
It is obvious that things such as
$network (which seems to refer simply to the possibility of a physical link, i.e., not an actual dhcp lease, etc.) and
$local_fs would be provided by the kernel during boot, but I cannot find any documentation which explains how they would be considered pre-defined as available or not available by the init system.