A rural route is a postal delivery that follows a road rather than a grid of named streets, so an address on it is the route's number and a box on it — RR 2 Box 152 — instead of a house number on a street: RR (rural route), HC (highway contract route), RFD and Star Route in the United States, RR, SS (suburban service) and MR (mobile route) in Canada, and seven abbreviations Australia Post uses for rural and community delivery, RMB, RSD, RMS, MS, CMB, CMA and CPA.
What Spaw answers
address_type reads rural_route in those three countries. Every one of these words counts only at the start of a line and followed by a number, because RR, HC and MS turn up in ordinary text and a bare "Rural Route" could be a road's name.
The type itself carries no risk weight: a rural route is how post reaches homes where there is no street delivery, and it is not a box. It stands in for the street line, so an address written as a route, a town and a postcode is valid and missing_street does not fire; the route line itself stays in street, and po_box stays empty.
What it cannot confirm
No register of buildings lists a route and box number — the registers list civic addresses — so the answer can confirm the town, the postcode and the region, but never the delivery point itself, and exists stays null. Australia is the exception today: G-NAF is a register whose coverage is complete, so where it is installed a route line it does not list answers exists: false and scores not_in_register, +20. Read that as "G-NAF lists no civic address written this way", not as proof that the route is not served. Where a property also has a civic address, that is the one a register can find.