The Z of Zip Code refers to the ZONE number included in a Zip Code. Zip Codes are used by the United States Postal Service to identify specific locations for mail delivery. Each Zip Code is made up of five digits and an optional four-digit add-on code that further identifies a location. The first digit of the Zip Code is the ZONE number, which can range from zero to nine. The ZONE number helps the Postal Service determine the general region where the mail will be delivered. ' The term 'The Z of Zip Code' refers to the ZONE number included in a Zip Code. Zip Codes are used by the United States Postal Service to identify specific locations for mail delivery. Each Zip Code is made up of five digits and an optional four-digit add-on code that further identifies a location. The first digit of the Zip Code is the ZONE number, which can range from zero to nine. The ZONE number helps the Postal Service determine the general region where the mail will be delivered.