ZIP+4 codes are extensions to original five digit zip codes that include more specific geographic locations. They are designed to aid in the sorting and delivery of mail.
The original five digits of the zip code indicate a specific part of the country. The sixth and seventh digits include more specific areas, such as a city block or a group of apartments. The eighth and ninth digits are even more specific, identifying individual P.O. Boxes, streets, or even representing a whole floor in a building. Companies such as UPS and FedEx have adopted the ZIP+4, though it was originally created for USPS.