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Publication 28 - Postal Addressing Standards >
2 Postal Addressing Standards > 27 United States Postal Service Addresses
Mail addressed to the United States Postal Service must contain a complete address.
Addresses may be located in the ZIP+4 file as street, firm, or Post Office Box records (record type S, F, or P, respectively). Most records carry the –9998 add-on code. However, many areas use different add-on codes.