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.
For example: a company ‘A’ which uses SAP MM module and and company ‘B’ which uses SAP SD module have a business transaction.