I believe the consumption is used when a credit note is processed - you do not issue stock at this point, therefore you can differentiate in the accounts the packing slip processing by sales order or credit memo.
Addition: It is not always updated in Case of Sales, If u do not select Financial posting and post a Purchase Invoice this will get posted. Which specifies nexpensing inventory against Purchases. The way it does in case of posting inventory against Jobs in NAV.
Issue is always a credit to Inventory i.e. in case of Sales Shipment, Consumption and Transfer from or any thing going out of inventory.
As far as Packing Slip accounts are concerned they are just interim accounts gets updated at physical update and reversed at the time of Financial posting.