To get to this page, to to Services → Webmail Server → Webadmin Settings

The Settings start page option simply indicates whether you are taken to the Status page or Users page after logging into the admin settings.

The Admin Area Access Restrictions are a second set of IP Access Restrictions which restrict which IP addresses/users can access the Admin settings (obviously users need to be Administrators too). Users need to first have access using the base IP Access Restrictions to be able to log into Webmail at all, and then have access through the Admin Area Access Restrictions to be able to access the VPOP3 Admin settings. If this list of restrictions is empty, then any administrator who can access the Webmail service can also access the Admin settings.