PayPal Payment Pro - 应该使用哪个字段来确定订单的确定答案

I am implementing PayPal payments pro for a client at the moment, and therefore I am configuring the IPN service to confirm the purchase on the clients server. What I can't work out is which field should be used to "carry" the id of the order to be passed back to the IPN handler?I am implementing PayPal payments pro for a cli