The relationship between ZIP Code and ZCTA can be determined fully only by comparing individual block-geocoded addresses to the ZCTAs. This process is quite involved. Some examples of why the process can become quite involved are as follows: ZCTAs follow census block boundaries. In contrast, USPS ZIP Codes serve addresses with no correlation to census block boundaries; therefore, the area covered by a ZCTA may include mailing addresses associated with ZIP Codes that are not the same as the ZCTA.
A ZCTA may include a mailing address with a unique or PO Box ZIP Code that is ineligible to become a ZCTA. Addresses with PO Box ZIP Codes generally cluster around a post office, but they may be widely scattered across several ZCTAs. Consequently, the relationships that exist between ZCTAs and ZIP Codes can become quite complicated, so that within the boundaries of a single ZCTA there may exist several ZIP Codes; likewise, within the boundaries of a single ZIP Code, there may exist more than one ZCTA.
Some addresses included in the census and used to define ZCTAs (typically in rural areas) have incomplete or, in some cases, no mailing ZIP Code, thus making it difficult to determine the full extent of the relationships between ZCTAs and ZIP Codes.