File: //usr/share/webmin/virtual-server/help/config_appendadmin.html
<header>Include domain name in extra admin usernames?</header>
This option selects how usernames for domain extra admins will be generated. By
default, Virtualmin will use the base username requested, and report an error if
there is a clash with any other domain. However, selecting "Yes" here will
cause the username to have the first part of the domain appended or prepended,
as determined by the "Domain name style in usernames" config option. <p>
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