A shipment reference is an identifier that you define by completing the "reference" fields during the shipping process. This reference could be a purchase order (PO) number, a customer number, a company name, a Bill of Lading number, or a phrase that identifies that shipment -- such as "10 crates" or "gift for Mum." Shipment references can be any combination of letters and numbers up to 35 characters.
For UPS Mail Innovations® shipments, a shipment reference is a unique package ID that you assign through bar-coded information for each of your mail pieces. This information can be up to 30 characters in length and be alpha and/or numeric with no spaces or symbols. Package IDs should not be reused for at least six months.
These reference numbers are used to identify and track shipments as they move through the UPS Mail Innovations network and the United States Postal Service®, which makes the final delivery.