Hi,
in /usr/share/doc/base-passwd/users-and-groups, base-passwd provides a
list of system users and groups that come with base-passwd.
On the Debian Wiki, there is https://wiki.debian.org/SystemGroups, which
does basically the same, but also for system groups that get installed
by certain packages.
Care should be taken that these two (probably equally authoritative)
sources dont diverge too much from each other, and they both should
reference the other and vice versa.
I'm sorry, I don't have enough time to take that care myself beyond
pointing the issue out to the package maintainers of base-passwd.
Greetings
Marc