A post office box with physical address is a service the U.S. Postal Service provides that enables post office box holders to use the street address of the post office to receive deliveries. This enables post box holders to receive deliveries from private carriers that don't deliver to post boxes.
Officially known as P.O. Box Street Address, or PBSA, this service is optional for post office box holders who want to receive packages and other deliveries from UPS, FedEx and some merchants that require street addresses. Instead of writing the P.O. Box number on the address line, box holders signed up for PBSA can alternatively write the street address of the post office followed by the number symbol (#) and the number of the post office box. They can also continue to use the normal post office box address for other mail.
Although private mailbox services also offer street address service, USPS post boxes are less expensive and more permanent alternatives. Post box holders can only use the number symbol before their post box number and not suite, apt. or room, or the delivery may be returned to sender. The box holder can only use the address for delivery of mail and not as a residence address on legal documents.