Production BOM Version.
On Released Prod. Order, the Prod. BOM Version used is:
Starting Date of Prod. BOM Version <= Due Date of Prod. Order
Why does it look for Due Date? Why not Starting Date of Prod. Order? Because I think the Product Designer would set the Starting Date to the date it will be used (Starting Date of the Prod. Order), when producing; instead of the production will be finished.