Secondary address unit designators, such as APARTMENT or SUITE, are required to be printed on the mailpiece for address locations containing secondary unit designators. The preferred location is at the end of the Delivery Address Line. The pound sign (#) should not be used as a secondary unit designator if the correct designation, such as APT or STE, is known or is shown in the ZIP+4 file.
The most common unit designators are:
If the pound sign (#) is used, there must be a space between the pound sign and the secondary number.
If all Delivery Address Line information cannot be continued in the Delivery Address Line above the city, state, and ZIP Code, place secondary address information on the line immediately above the Delivery Address Line.