This occurs in accounting when payment is processed. After the item has been picked, accounting processes payment. Once payment has been processed, the order is closed and the shipping department or the warehouse is notified. When you close an order, you subtract the items from the on-hand inventory. The two inventory counts follow each other closely, but are subtracted at different stages during the inventory management process.
Available inventory is depleted when the item is picked.
On-hand inventory is depleted when payment is processed and the item is ready to be shipped. This is a critically important component of order fulfillment. The regularly scheduled aging order report tells you what items have been picked but not shipped. If an item is picked but not shipped after a set time, investigation is needed to expedite the order. This is referred to as an item being pulled but not closed.