→I'm Still Confused -- What are these things?
When you assign additional user ids or group ids to a user, they become reserved for use exclusively by that user. This
means that the range of ids you assign are no longer available
for use by other users.
It also means that the user to which they are assigned now 'owns' these ids, so that the user can change ownership of files to be owned by these ids, and run processes under these ids.