The last four digits of a zip code identify a specific area in locations that receive a high volume of mail delivery. The digits may correspond to a specific apartment or commercial building, city block or other small geographic space that houses a relatively large number of addresses.
The additional four digits were added to the ends of zip codes beginning in 1983, primarily to help with sorting in the post office. The first and second digits of the additional four numbers correspond to a delivery sector, such as a group of streets or a high-rise building. The third and fourth numbers correspond to a delivery segment, such as a side of the street or a floor of a single building.