The format, and the words that break it
Section 241 sets the pattern: "The rural route number on a mailpiece must be standardized as follows: RR ## BOX ##", followed by a plain instruction not to use the words RURAL, NUMBER, NO. or the pound sign. Section 242 says a leading zero before the route number is unnecessary. Section 243 allows hyphens in the box number only when the ZIP+4 file carries them. Section 244 converts the older RFD and RD markings to RR. Section 245 keeps town and street names off the line altogether, and asks that any secondary name go on the line above. All from pe.usps.com, checked 2026-07-28.
Highway contract routes follow the same shape
Section 251 gives HC ## BOX ## for highway contract routes, again as a single standardized line, and again with a list of words to leave out: HIGHWAY CONTRACT, ROUTE, NUMBER, NO., STAR ROUTE and the pound sign. The Postal Service address files encode which kind of route serves an address in a four-character carrier route identifier, where R marks a rural route, H marks a contract route and C marks city delivery, per the Address Information System Products Technical Guide on postalpro.usps.com, checked 2026-07-28.
What this does to carrier choice
A route-and-box address has no street for a driver to navigate to, so private carriers ask for the physical address of the property instead. Most rural households have one, often assigned for emergency services, and it can be used on a UPS or FedEx label while post goes to the box. Two other things ride along on a rural destination: UPS defines a residential delivery as one "to a location that is a home or a home-based business in a residential area, such as a neighborhood", per ups.com, checked 2026-07-28, and remote ZIPs commonly pull an area charge on top of the base rate.