A PO box is a numbered box rented at a postal facility, where mail is held for collection instead of being carried to a street address. Every postal service has its own name and its own written form for one — Post Office Box, Boîte Postale, Postfach, Casella Postale — and 52 countries' naming rules are held here.
What Spaw answers
is_po_box is the flag and address_type reads po_box. The risk score carries +30 — but only where a box is unusual. In the nine countries where a box is the ordinary way to receive mail (the Gulf states, Jordan, Lebanon and Hong Kong) the weight is deliberately zero, because scoring somebody for the way their country delivers post is not a risk signal. exists stays null either way: a box with no street behind it is not a building, and no address register lists it.
What it means
A PO box is a real, deliverable address that names no premises. That is exactly right for a small business collecting post and exactly wrong for a delivery that needs a door or a check that needs a residence — which is the difference the weight is trying to express, and the reason the flag is reported rather than the address refused.
A box rented from a private company rather than the postal service is a different thing with a different weight: see private mailbox.