An "order number" is a unique code or identification number given to an order made by a customer. It helps businesses track and locate specific orders among many others. Full definition
Some might need to issue a Purchase Order number before you send over your invoice; others may request there is a certain company name included on the document. (creativeboom.com)
If the original order number for the gift can not be located, a gift certificate will be provided. (peekaboobeans.com)