Stages for execution of a purchase order received

Big industrial organisation, always at initial stage demand quotation frm vendors and after finding it suitable draft a purchase order to give a reply for the said quotation... The po is a legal document which deliver the specification of the services or goods to be delivered. The po stands alone as a contractual obligation about confirmation of quotation and a basis for the vendor to demand their payments. Po in itself is a document, so no further document is needed. For vendor it is a basis for making the invoice as the po is the approval of the quantity and rates.