It is whatever you have it set as in the Store Cart Summary in Database server behavior. It has a default of OrderID I think, but check the confirm.php page to be sure.
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.