Debian typically uses code names to refer to its releases, starting
with the Toy Story character names used (mostly) instead of numbers.
The “Buster” release is due on July 6 and you will rarely hear it
referred to as “Debian 10”. There are some other code names used for
repository (or suite) names in the Debian infrastructure; “stable”, “testing”,
“unstable”, “oldstable”, and sometimes even “oldoldstable” are all used as
part of the sources for the APT
packaging tool. But code names of any sort are hard to keep track of; a
discussion on the debian-devel mailing list looks at moving away from, at
least, some of the
repository code names.

Source: LWN.net – [$] Debian and code names