A completed delivery is simply a physical delivery that has occurred. An example of this would be a delivery that has been put onto an out-turn order where grain has been delivered but there has been no change in ownership of the grain.
A delivery will have a transferred status when there has been a change of ownership. For example, if a sale contract is drawn up, the grain is being sold from one counter party to another and therefore the ownership of the grain has changed. The delivery becomes 'transferred' as the grain changes ownership once the delivery has occurred.