1. Legacy notation
This is the same as in "sources.list" except currently only the "file"-protocol is supported and the available architectures have to be appended.
deb file:/debian woody sid contrib main i386 powerpc deb-src file:/debian woody sid contrib main i386 powerpc
This describes the dists woody and sid with their components contrib and main and restricts it to arches i386 and powerpc. Both packages and sources where described and the base directory is "/debian". The location for any control file is "dists/<dist>/<comp>/(binary-<arch>|source)"
2. Standard notation
This describes each distribution on a single line with its base directory, the location path and the name. For compatibility reasons the location path and the name has to be identical to the "sources.list" definitions or the name has to correspond to this naming schema "<dist>_<comp>_<arch>". The naming schema is better described in "distsync.txt".
deb file:/debian dists/woody/main/binary-i386/Packages
deb-src file:/debian dists/woody/main/source/Sources
This describes just the woody/main distribution for i386 (packages and sources).
deb file:/debian pool/woody_main_i386
deb-src file:/debian pool/woody_main_source
This allows to have the control files within the pool itself and all could be in the same place. The control file could be right in the base dir. The name of the control file itself describes the distribution. To make such a definition compatible with "sources.list", a corresponding shortcut has to be placed in the correct location.