The first two digits of the ZIP Four identify an area known as a sector and the last two digits identify a smaller area known as a segment. Together, the four digits identify geographic units such as a side of a street between intersections, both sides of a street between intersections, a building, a floor or group of floors in a building, or a firm within a building. For mailing addresses, it could also identify a span of boxes on a rural route or a group of post office boxes to which a single USPS employee makes deliveries.
The vast majority of Infogroup records have a ZIP4. However, some addresses cannot receive a ZIP4 because they reside in a single ZIP city and will not provide a location or mailing address, reside in a new rural development in which the USPS has not completed its analysis such as a rural route delivery or will only provide a general address such as "Corner of 84th & Main Street".