In the POSIX Definitions there is specific section 3.424, User Database, which defines what information should be related to each user ID. However through out the document, I couldn't find (at least not via simple Ctrl+f search) mention of either
root nor any of the typical system usernames, such as
sys. There doesn't seem to be mention of these in newgrp utility definitions either, to which the 3.424 refers.
This begs the question: does POSIX require existence of certain system-level usernames or UIDs ? Is
root user even required to be
root, or is it theoretically possible to have POSIX-compliant system where the most powerful system account is not called
root or has non-zero UID?