After the first number in a ZIP Code is assigned, the USPS assigns the next two numbers according to a city. If a region has a main town or city, the USPS will often assign it the first ZIP Codes. After that, the ZIP Codes will proceed alphabetically.
The first three digits of a ZIP Code together usually indicate the sectional center facility to which that ZIP Code belongs. This facility is the mail sorting and distribution center for a zone or area. Some sectional center facilities have multiple three-digit codes assigned to them.
The fourth and fifth digits of the ZIP Code represent the area of the city or town.