Extended Zip Code functionality.
A ZIP+4 code uses the basic five-digit code plus an additional four digits to identify a geographic segment within the five-digit delivery area, such as a city block, a group of apartments, an individual high-volume receiver of mail or any other unit that could use an extra identifier to aid in efficient mail sorting and delivery. Use of the plus-four code is not required except for certain presorted mailings. In general, mail is read by a multiline optical character reader (MOCR) that instantly determines the correct ZIP+4 code from the address and — along with the even more specific delivery point — sprays a Postnet barcode on face of the mailpiece that corresponds to 11 digits. This technology has greatly increased the speed and accuracy of mail delivery and, in turn, kept costs nearly constant for over a decade.