Secure Area= Area between WTMD and the end of the CP

Sterile Area= Area beyond the CP

UnSecure Area= Before WTMD

And how would a passenger getting up and leaving with his/her belongings and leaving through the exit= "interferring with screening"?

They have every right to refuse screening at any point during the search. But they have to talk to a officer if they do so. If they just get up and leave the checkpoint through the exit, LEO's have to be notified but thats about all we can do as TSA.