what is the difference between open sales order and back orders?
A open sales order is the one which is not yet delivered that means u have only created a sales order and saved it( create a sales order, save it and see the document flow, the order shows as open)
Back order is also an order which is not completely fulfilled. may be because of the target quantity or because of not meeting the requested delivery date.
in this case you will pull back the required quantity from other sales order(open sales orders) this is done using T.Code CO06.