Zip Codes are five-digit numbers established by the United States Postal Service, for the purposes of identifying a location and routing the mail to that location. Zip+4 is the five digit Zip Code, which indicates the state and post office or postal zone, plus four digits as an expanded code which denotes the box section or number, portion of a rural route, building, or other specific delivery location. There are approximately 41,000 Zip Codes in the US, and the boundaries of Zip Codes do not typically match with the boundaries of Block Groups or Census Tracts. The Zip Code may split a Block Group.