Format for usernames that include domain
When a username has the domain name appended, this option selects the format.
Not all options are available on all OS platforms, as some characters are not
allowed in usernames. Whether the domain name is the full username, i.e.
"virtualmin.com", or just the first part, i.e. "virtualmin", is dictated by the
Domain name style in usernames option on the Module Config page.