@tom.willett this related post contains some information on releasing backorders aka handling backordered quantities for Sales Orders.
Generally, a Sales Order will contain a backordered quantity if the Available Quantity for the Item in the Inventory Commitment Overview or Item Balances is not sufficient for the Requested Quantity of the given Item, Organizational Unit, Job or Lot. See Inventory Management for more on determining availability.
A backordered quantity will not be eligible for further fulfillment processing until more quantity becomes available or other Sales Orders containing the item are canceled or their commitments re-prioritized behind another. This can be done manually on the lower priority Sales Order by using the row action “Undo Commitments”.