When you first place an order, it is an open order. Once a day, open orders for available items enter processing. At that point, your open order for an available item becomes a processing order. Payment is then sought for the processing order.
After payment is received, the processing order either enters processing for shipment (if you've selected Ship When Available/Ship Now for the handling method) or enters Private Warehouse (if you've selected Private Warehouse for the handling method).