A value chain consists of interconnected activities that add value and competitive advantage as inputs are converted to outputs. It typically includes inbound logistics, operations, outbound logistics, marketing and sales, and after-sales service, supported by purchasing, R&D, human resources, and infrastructure. Value chains are supported by demand and supply chain business models and manage transactions between multiple enterprises by delivering products from producers to end users. They must manage relationships with customers and direct sales as well as channel partners and suppliers using different management approaches.