Let us now look at the way the ZIP codes work. It was initially registered as the trademark for the USPS. The style is all caps though lowercase and sentence case can be used as seen in some publications. The first digit represents a group of States within the US. The second number and the third digits together represent the region that a large city or the group of states. The fourth and the fifth numbers represent the group of delivery address within that area or region. The main towns in every region always get the first ZIP codes for that area. The other ZIP codes now depend on the numerical order or with alphabetical order.