Shipping cost document

Hi Guys,

I have one doubt on the Shipping cost document.

As I understand that there is one customization at Item category for Generate PO (for automatically generating purchase order) under "Shipment Cost Types and Item Categories". I want to know from where does the Purchase order document type is taken and from where does the system automatically pick vendor. If this going to automatic generation of PO.

Regards,

Mauryan.