The REA pattern lacks reusability and extendibility. Its stock-flow relationship with economic resources. A sale results in the outflow of products, a purchase results in the inflow of products. Its control relationship with economic agents outside of the firm.
Customer is the external agent for sale and vendor is the external agent for purchase.
Its duality relationship with another economic event where the increment events are paired with decrement events in a give-take relationship. Sale is paired with cash receipt and purchase is paired with cash disbursement. The REA pattern lacks reusability and extendibility. Extendibility is the ease of adapting the accounting information system to changes in the economic activities of a company. An example will help to clarify this statement.