The ZIP+4 code is the normal ZIP code plus four extra digits that indicate a specific location within the delivery area. Examples include an office building, apartment building or city block. The purpose is to make mail sorting more efficient, although the last four digits are not required for delivery.
The first two digits of the extra four numbers at the end of the ZIP code narrow the address down to delivery sector, meaning a certain part of the delivery area such as a group of streets or buildings. The last two digits narrow the address down further to a delivery segment or specific part of the delivery sector. Examples include one floor of a multi-story building or a certain side of the street.
Companies such as UPS and FedEx have adopted the ZIP+4, though it was originally created for USPS.