A reference number in Speed Post is a unique identifier that serves as a tracking number for a package. It can also be called a barcode or a consignment number. The reference number is usually a 13-digit alphanumeric code that uniquely identifies the shipment. It can be found on the Post Office receipt given to the sender at the time of booking. The reference number is required to track a Speed Post package. If the sender has lost the receipt or tear-off label, they can contact the Post Office customer service for assistance. The reference number is different from the postal tracking number or the Post Office tracking number.