Q. What is a Zip Code Tabulation Area (ZCTA) and how it is calculated?

"ZIP Code Tabulation Areas (ZCTAs) are generalized area representations of United States Postal Service (USPS) ZIP Code service areas.

The USPS ZIP Codes identify the individual post office or metropolitan area delivery station associated with mailing addresses. USPS ZIP Codes are not areal features but a collection of mail delivery routes.

The term ZCTA was created to differentiate between this entity and true USPS ZIP Codes.  ZCTA is a trademark of the U.S. Census Bureau; ZIP Code is a trademark of the U.S. Postal Service."

See the Census Bureau site for more details.