Virtual sites have been extended with the ability to prefix usernames with a 5 character alphanumerical prefix.
If a prefix is set (and only if), then all (new) users of the site will be named <prefix>_<username>.
If a server admin sets a prefix, a siteAdmin cannot remove it. So it forces the siteAdmin's to a naming scheme of their users which can make the life of a server admin a bit more convinient.
Additionally the "Site Prefix" will also be used to prefix the name of a new Mailman mailing list. So if the site prefix is "fwst", then a newly created Mailman mailing list named "support" will be named "fwst-support". If no "Site Prefix" is given, Mailman mailing lists will be prefixed with a (more or less random) number instead.