How is the plus 4 zip code determined?

A U.S. postal ZIP Code has six components:

1. Regional prefix, 3 digits, identifies a major city or a part of a state
2. Local ZIP Code, 2 more digits, identifies a town or a part of a city
3. Sector, 2 more digits, identifies a part of a 5-digit ZIP Code area
4. Segment, 2 more digits, identifies one specific address, up to a maximum area of one side of one block of one street
5. Delivery Point, 2 more digits, usually the last two digits of the street number or P.O. Box number, identifies one specific delivery address
6. Check digit, 1 more digit, used to verify the data

For example, in 12345-6789-01-4, 123 is the regional prefix, 12345 is the local ZIP Code, 67 is the sector, 89 is the segment, 01 is the Delivery Point, and 4 is the check digit. However, the Delivery Point and check digit are included only in the barcode, not normally written out in human-readable form. Thus, you would write the address as

John Doe

3701 Any Street

Anytown, XQ 12345-6789

and the USPS would look up the Delivery Point and calculate the check digit in order to print the bar code.