Q:

What is the default 'backup' user for?

I am setting up multiple-machine backup plan and I was hoping to use a user called 'backup' on each machine as the backup destination. 
My problem is, there is already a user called 'backup' on my machine.
What is it for, and can I hijack it for my own purposes?
root@frodo:~# useradd backup
useradd: user 'backup' already exists

A:

I can confirm it's in a default installation, see the /usr/share/base-passwd/passwd.master file provided by the base-passwd package.
According to the documentation from that package, it is used for backup accounts without requiring full root permissions (which is available at /usr/share/doc/base-passwd/users-and-groups.txt.gz, /usr/share/doc/base-passwd/users-and-groups.html and online):

backup
Presumably so backup/restore responsibilities can be locally delegated to
     someone without full root permissions?
HELP: Is that right? Amanda reportedly uses this, details?

Note the keyword locally, for remote backups you have to enable a login shell first. You are free to use it for your own purposes, but note the above guide lines. Do not grant sudo policies for example that would allow the backup user to escalate its privileges to root.

