It is a requirement by some retailers that when an item is back ordered, the change Acknowledgement sent has both IC and IB ItemStatusCode sent for this order. 

You will receive the following error message from SPS


A line item with a LineItemAcknowledgement [ItemStatusCode = IB] must have a LineItemAcknowledgement[ItemStatusCode = IC] also present  


The retailer setting Ensure item change on back orders is the solution.