Available to Promise (ATP) is a business function that provides response to customer order enquiries based on resource availability. It generates available quantities and delivery dates. ATP is calculated as On-hand + Supply - Demand, where On-hand is quantity on hand, Supply is planned orders and receipts, and Demand is sales orders and component demand. Companies define ATP rules and attach them to items and bills of materials to tailor the ATP calculation to their business needs. ATP inquiries are performed using the ATP Inquiry window.