The last four digits of a ZIP code indicate a more focused area than the typical five-digit ZIP code. The first two numbers indicate the delivery sector and delivery segment.
The delivery sector is a specific group of streets, buildings or post office boxes. In some cases, if a building is large, the numbers may only represent that building. The delivery segment represents a floor in an apartment or office building, a specific side of the street or even one department within a big office building. The four additional numbers began to appear on addresses back in 1983 due to the strain on the former ZIP code system.