The key components of the SAP SD module include master data, basic functions, sales processes, shipping and transportation, billing, and sales support. A sales organization is the topmost organizational unit in SD and represents selling and distribution transactions. A distribution channel depicts the paths products take to reach customers after being sold. A distribution chain is a combination of a sales organization and distribution channels. A sales order is a contract between a sales organization and customer for supplying goods and services over a specified time frame and quantity. Special sales document types include rush orders, credit memos, return orders, and free of charge deliveries. The consignment stock process allows stock to be stored at a customer's site and billed only as consumed.