As the name suggests, Open Purchase Order Quantity is the total quantity yet to be received on a purchase order in SAP MM. Thus, if a PO purchase order was made for 100 numbers of a particular material. Let us suppose, there were two goods receipt for this purchase order say of 20 numbers and 45 numbers. Now the open purchase order quantity for this order would be 35 (100-20-45). Open purchase order quantity is nothing but the difference between the quantity ordered and quantity delivered till date.
SAP MM material management automatically updates the open order purchase quantity when goods are received on a purchase order or when a PO is altered. When a user makes an entry for goods receipt, SAP proposes the open purchase order quantity as the quantity received. SAP MM calculates underdeliveries or overdeliveries based on the goods receipt and the open purchase order quantity.