There are so many links between SAP SD and SAP MM. For example, Material Management(MM) is required to create inbound and outbound updates for sales and distribution. Similarly, price details and the availability of the product are taken from MM but are controlled by the sales and distribution module.
Orders placed by the customers should be extended to the sales area in an organization, or else it gets difficult or impossible to transact with the material. This shows that these modules are interconnected to each other and all the other modules are interrelated.
