virtdomains: userid?

[Bill Kearney]

The docs for /etc/imapd.conf read:

virtdomains: off
            Enable  virtual domain support.  If enabled, the userâs domain
will be determined by splitting a fully qualified userid at the last '@â'
            or '%' symbol.  If the userid is unqualified, and the
virtdomains option is set to "on", then the domain will be determined by
doing  a
            reverse lookup on the IP address of the incoming network
interface, otherwise the user is assumed to be in the default domain (if
set).
            Allowed values: off, userid, on

What actions does the "userid" value trigger?   In testing I can't see that
it does anything.  Are the docs correct?

Thanks,
-Bill Kearney