My products are assigned a serial number by my manufacturer and then shipped to me. When a customer orders my product(s), I take product from inventory and ship it to them. I need to track the serial numbers that shipped for that order. If they ordered three items, then I need to assign a unique serial number with each item ordered. If a customer called me two years from now, I'd ask them for the serial number and then I would want to pull up their order number.