Phil,

The "logged out default profile" is the profile which gets loaded on the phone when a user loges off. So you have the option to create logged off device profiles instead of configuring the dn/pt/css settings fixed on the phones.

here is the text from the "help for this page"

If the device profile is used as a logout profile, specify the login user ID that will be associated with the phone. After the user logs out from this user device profile, the phone will automatically log in to this login user ID.