Who maintains the list of assigned UIDs?

[VA1DER]

When a new system user is required for a package, who is the authority for assigning it? I am assuming there must be some list maintained somewhere to avoid UID conflicts. Is that list published somewhere?

[neuraleskimo]

When a new system user is required for a package, who is the authority for assigning it? I am assuming there must be some list maintained somewhere to avoid UID conflicts. Is that list published somewhere?

It seems you posted the same question here: http://forums.debian.net/viewtopic.php?f=10&t=142378
GarryRicketson gave you good advice and I would suggest following the links he gave. To his excellent advice, let me underscore the something he wrote

There is no real technical difference between the system and regular (normal) users. Typically system users are created when installing the OS and new packages.

Yes, root is always 0 and most of the users from the initial install end-up the same, but you are the authority. Let me give you a concrete example: on one of my systems gdm has UID 117 and on another system sddm has uid 117 (both are fresh installs of Debian Buster from the same alpha media).

[p.H]

AFAIK, system users have no predefined UIDs. System UIDs are dynamically assigned when system users are created.
Same with system groups.