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The ZIP+4 code is not required, but it aids the post office in additional sorting of mail. A ZIP+4 code may correspond to a city block, group of apartments, or an individual high-volume receiver. It is also common for each PO Box number to correspond to a unique ZIP+4 code. Sometimes, several PO Box numbers are grouped into the same ZIP+4 code by using the last several digits of the PO Box number. This method isn't a universal rule though so the ZIP+4 must still be looked up for each PO Box.