Postal Code List Zones (ZCTAs)
The ZIP Code List area was developed by the U.S. Census Bureau. Their purpose is to convey statistics of areas familiar to most citizens. However, ZCTAs are not exactly the same as zip codes.
As mentioned above, it is difficult to precisely define the geographic area covered by a zip code. ZCTAs were developed to address some of the difficulties encountered in assigning a zip code to an area and to precisely define a geographic area. Also, ZCTAs are not updated as frequently as zip codes. Generally, they are updated every 10 years for the census.
Match zip codes to states, counties, and cities
Remember, zip codes are there to make mail delivery easier. They don't fit existing boundaries like cities, counties, or even states. If it were more efficient for the postman to drive across state lines to deliver mail, the zip code "border" would cross the state line.
ZIP codes don't usually cross state lines, but some do (65733, 71749, and 73949 are good examples). A full list of interstate zip codes is available as our list of U.S. zip codes.
It gets more complicated when trying to assign a zip code to a specific county (as much as 25% of inter-county lines), congressional district, metropolitan area, time zone, area code, etc. The edges of the border often overlap. For our purposes, when the County Free ZIP Code Database downloads, we usually list the areas where that ZIP code is most common, or list multiple areas if more than one area exists in that ZIP code.
For cities, the task is a bit more complicated. USPS doesn't always use the city where the zip code is located. It is more common to assign cities to zip codes. The city is usually the name of the main post office. For example, nearly all zip codes in St. Louis County, Missouri, have a city of St. Louis, but a more accurate description is the name of a smaller city in which they are located.