For the one who had known SAP MM Purchasing before he learned SAP SD module (or the opposite), he might see that there are similiraties between these two modules. What I said is about the business process flow. A business transaction or document in one module in a company may trigger a business transaction or document in other module in other company.
A user in company A creates a purchase requisition (PR) for a material (t-code: ME51N). The buyer (procurement department) of that company processes that PR by converting it to some Request for Quotations (RFQs) (t-code: ME57) that are printed and sent to some potential suppliers/vendors including company B.
When the sales person in company B receives the RFQ, he creates an inquiry in SAP SD module (t-code: VA11). The inquiry refers to the RFQ, he may record the RFQ number in the ‘PO number’ field of the inquiry document.
Then, the sales person will analyse the request from company A. When he has determined the price, delivery terms, term of payment, etc, he will create a Quotation in SD module (t-code: VA21). The quotation refers to the inquiry created before.
Then the quotation is printed and sent to company A. The buyer of company A will receive the quotation that refers to their RFQ number and input the information of it to SAP MM module through ‘maintain quotation’ transaction (t-code: ME47). The buyer may receive several quotations from several suppliers. He will compare the quotations (t-code: ME49) and decide from which supplier he will buy the material. Let say, he will buy the material from company B. The buyer will create a Purchase Order (PO) in MM module (t-code: ME21N). The PO is created referring to the quotation that has been maintained before.
The buyer will print and send the PO to the company B.
The sales person in company B will receive the PO. He may compare the PO with the quotation he sent to the customer before. Based on that PO and referring to the quotation, he can create a Sales Order (SO) in SD module (t-code: VA01). When the material is ready to be delivered to the customer, he can create an Outbound Delivery document in SD module (t-code: VL01N). A delivery document triggers warehouse personnel to prepare the material to be sent to customer. Then warehouse personnel will post the Goods Issue transaction that refers to the outbound delivery document (t-code: VL02N). He can print the Goods Issue document and send it with the material to the customer as a delivery note.
When the customer’s warehouse personnel (in company A) receives the material, he can post the goods receipt based on PO and delivery note sent by company B (t-code: MIGO_GR). He can print the goods receipt document and give it to the company B.\
After receiving the goods receipt document, finance personnel of company B can create a billing document in SD module refers to the outbound delivery created before (t-code: VF01). Then he can send an invoice document to request payment from company A. He usually needs to attach the PO and goods receipt document with the invoice sent to the customer.
The account payable personnel of company A receives the invoice and other attached documents. He performs the three way match by checking the match of PO, goods receipt, and invoice document. If the 3 way match is OK, then he can post an invoice receipt document in MM module (t-code: MIRO).
Then the payment process will be performed in FI module.