Many businesses keep track of purchases with an internal tracking system that issues purchase order numbers. These numbers are commonly supplied to vendors to be included on invoices and packing slips to aid receiving and accounts payable departments with cross referencing vendor order numbers to the internal accounting process. I can't think of a single major vendor that I deal with that doesn't supply a field of this type in some section of their checkout process. You can see what I mean at www.mcmaster.com/#orders,