recommend a tag for user accounts in computer documentation?

["David O'Brien" <david.obrien@redhat.com>]

Hi
I'm looking for recommendations for a suitable tag for user account
names. By way of example:

"Log in as root and run the following command."
"This process runs as the user nobody on Solaris systems."

We're currently using docbook 4.3

Thanks a lot.
-- 
/David

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