Backorder Qty is the amount of an Open Qty that you chose to ignore when Released.
Pictured, the Backorder has been keyed in as 7 and the Pre-release checkbox is clicked, which calculates Release to be 3. Saving the order as Placed will set the item Released.
Backorder Qty can be used without Pre-release to calculate when the order is Released. An option to link the Backorder Qty to an open Purchase Order may be added in time.