Debian updating packages
Much of the original development of APT was done on IRC, so records have been lost.
The 'Deity Creation Team' mailing list archives include only the major highlights.
The APT pinning feature allows administrators to force APT to choose particular versions of packages which may be available in different versions from different repositories.
This allows administrators to ensure that packages are not upgraded to versions which may conflict with other packages on the system, or that have not been sufficiently tested for unwelcome changes.
APT searches its cached list of packages and lists the dependencies that must be installed or updated.
This can downgrade a package and render dependent software inoperable, so the user must be careful.
For apt, a repository is a directory containing packages along with an index file.