This is to help use Transfer shipments to facilitate inventory transfers. The process is as follows:
1. Create a shipment of type "TRANSFER"
2. Create items for the shipment items in the Shipment > [Items] tab
3. Create the inventory transfers you want
4. Create an ItemIssuance for each item on the shipment. This can be done by just calling createItemIssuance with shipmentId, shipmentItemSeqId of the shipment item.
5. Set the InventoryTransfer.itemIssuanceId to the new ItemIssuance. This links the inventory transfer with the shipment.
6. Now when the shipment is COMPLETED, the SECA will cause its inventory transfers to happen.