In practice, Debian (and Ubuntu) have fixed minimum system UIDs and GIDs

January 7, 2022

Debian, like many Linux distributions (and probably many Unixes)
has the idea of ‘system’ logins and groups as distinct from normal,
non-system users and groups. System users and groups are created
by Debian packages for their own use, for either or both of file
access permissions (such as making TLS keys accessible by group
‘Debian-exim’ so that your Exim can read them) or for processes and
daemons to run as. The UIDs and GIDs for these logins and groups
mostly aren’t preassigned; instead, the package gets the next
available UID or GID from the system UID or GID range. In theory
Debian allows you to control these ranges through /etc/adduser.conf, so
that you can (for example) alter them to avoid a range of low UIDs
or GIDs that