When the Advanced Stocks Management option is enabled in PrestaShop backoffice, 3 new fields/columns are displayed in Store Commander's interface:
Physical Quantity: Physically available stock
Usable Quantity: Stock that is usable for sale
Real Quantity: Real stock is made of stock that is physically available in a warehouse, to which we add the quantity presently ordered from suppliers, and from which we subtract the quantity presently ordered by clients and which have not yet been shipped. For instance, at a given time, a product can be available as a physical stock, but not available for sale because some client orders are still waiting for shipping. That same product can also have a supply order in progress, and thus not yet accounted for in the physical stock.
Source: http://doc.prestashop.com/display/PS15/Advanced+Stock+Management