F2 Billing type in VOV8

Friends,

In VOV8 Sales document type OR, I have purposefully removed Order related billing F2 and delivery related billing F2 under the Billing tab and processed the complete cycles from sales order to billing. Upon creating the invoice with VF01, I was able to make invoice without any errors. Now my question is "How system knows, that to create F2 billing type without in the sales document type ??

I have checked in Item category also but failed to locate any such field controlling the flow or suggesting the system to take F2 billing type.

Now another situation is, I have also tried by giving the delivery no: alone in VF01 and tried creating the proforma invoice, that is also processesd successfully, How ??

Where else this control lies in the system to make the flow in this way, could any one help me understand this please ??

Regards,

Venkat.