Item number controlled in the billing document

Hi Guys,

I had a question about the item number in the SD billing document. The billing is delivery related and in the cpoy control (transaction code:VTFL), The function 'Copy item number' is indicated. Then as per the configuration, the billing item number should copy the delivery item number. But now it copy the sales order item number. The case is that SO has only one item,number is 20. Delivery has one item number 10. Billing has the item number 10. Any one can tell me that why the billing item number is not the delivery number? Or any user exit is used to change the item number to copy SO item number?