They would use it as a reference number on their docket / invoice to you, most Company's use them to match up orders against price. Its a Purchase Order number....if you don't use the system yourself make up one for example with todays date 090708 or a random number
Most companies use a purchase order system in order to track goods from order to invoice.
If you dont use a purchase order system, either make up a number or just tell them to use your name as the purchase order.