Flow of PO module:

Requisition
  |
RFQ
  |
Quotation
  |
Purchase order

Requisition
It is prepared by emp wheneve emp needs the goods
In this we have 2 types
Internal: within org
External: from outside

Tables are :
po_requisition_headers_all
po_requisition_lines_all
po_req_distributions_all

RFQ
Once req is approved..we will prepare the rfq doc which will b delivered to the customer.supplier will respond for that with quotation

Quotation
It is a doc we receive from the supplier which contains supplier code details

Tables are:
po_headers_all
po_lines-all