According to the u. s. postal service the domstic mail zone includes

The National Zone Charts Matrix was developed to assist mailers in applying the proper postage rates to a mail piece. Postage rates for zoned mail are based on mail piece weight and the distance the mail piece travels using these zone assignments. The matrix is used to determine the distance that a mailpiece travels based on the ZIP Code where the mailpiece enters the mailstream (origination) and the ZIP Code™ that will deliver the mailpiece to the addressee (destination). The origination and destination of a mailpiece is refered to as “ZIP™ pairing”. Zones are designated as "local" or 1 through 9.