The United States Postal Service embarked upon a plan to transform the largely manually-intensive sorting and delivery of the mail to a fully automated system.
One of the keystones of this plan was the expanded use of ZIP+4 and bar code. With the advent of Advanced Bar Code, or Delivery Point Bar Code, mail can be sorted right down to the carrier route walk sequence. ABC adds the last two digits of the street address to the end of the ZIP+4 code to give an 11-digit number.
As of March, 1993 ABC became mandatory for anyone applying for automation discounts when bulk mailing.
