You can only use this function if you use
As of Release 4.6C, 'order quantity optimizing' is called 'order optimizing' and is assigned to component LO-MD-QO.
For information about Customizing for quantity optmizing, see Logistics General -> Quantity Optimizing and Allowed Logistics Units of Measure.
For more information about quantity optimizing, see the SAP Retail documentation (SAP Library -> Logistics -> SAP Retail), under Purchasing -> Order Optimizing.