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The four additional digits are called an "add-on code," and are used to denote a smaller area inside the main zip code. They are used when a small area receives a large volume of mail, to make sorting and delivery more efficient.
So they are not unique to a specific address then; 2 or more different addresses can have the same add-on code?
They are not unique to two different street addresses!
In other words, 2 or more different addresses can have the same add-on code, correct?
If they are in the same area, the zip code add on can be the same.
So the add-on code is then unique to a specific AREA then, correct?
A specific area in a city or town.
Yes. They are used to denote a smaller area inside the main zip code. This helps the postal service. deliver and sort mail!
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If a person sends a letter to someone and the street address, city, state, and zip code are all correct, but only the add-on code is wrong, will the letter still be delivered? Thank you.