When the zip code was introduced in 1963, it actually came with five digits. These five digits had their own representation that they stand for in the way mails are distributed all through the United States. The first digit points to a particular region of the country. Under each of these regions, we can have a group of cities that are grouped together.
The second and the third digits direct the mails to a particular city in the country and the last two digits of the five digit zip code point towards the particular town where the mail is to be delivered. This system was used by the United States postal services and there was a great deal of transformation in the postal service of the country.
This situation held sway until 1983. This particular year marked the introduction of another sets of digits to the pre existing five digits of the zip code. This addition then brought the total number of digits for the zip code to nine.
This addition to the zip code can be depicted in various ways. There are some people that prefer to call it "ZIP plus four" or the "plus four code". Some other sets of people do call it the "add-on codes" and it can also be called "add ons".
This addition to the zip code of course had its own contribution to the way in which mails are handled in the country. It had been used for the easy identification of the geographic segment within the area of the delivery of the first five digits.
With the addition of the extra four digits, the zip code can be used to send mails to groups of apartments within a city or even city blocks. If an individual receives so many mails in comparison with others, the extra four digits of the zip code can be useful in getting mails across to this individual directly. If anyone requires an extra form of identification beyond the normal five digit zip code, the extra four digits can be used to identify such a person.