On our packing slips, 
W.O means Work Order. Everything we get will have 
W.O. on it, but only shop jobs will be assigned a  number behind the letters 
W.O.Stuff ordered thru parts for walk in counter customers will not have a W.O. number, but if that part is ordered for a customer's machine in service, it will have a number assigned behind 
W.O. W.O. is the same as the job #. Virtually everyone who ships has some sort of system to assign a job #, Purchase order # or Work Order #. FedEx, UPS, etc. All their packing slips will have W.O. or P.O. but will not neccessarily have a # behind these prefixes. 
Sometimes, a shipping company like Central Freight will assign a W.O. # that is only for their own use. There may be 100 items associated with that #, but each could go to a different customer.
[This message has been edited by maryjane (edited 04-17-2008).]