Just to give a further example as to how this can be used...
PRFrameAdmin class comment explains:
====
This frame displays the site using the layout in /environment/admin,
rather than the default.
This allows a second seaside application to be set up which shows an
admin interface to the whole site.
This application can be configured to need authorization before viewing,
while the main site is public.
Thus, two (or potentially more) levels of security can be configured
without using Pier-Security.
===
Keith