Dear Hohn,
If you choose Immediate for the Delivery Priority, the sales order is treated as a rush order and is released to the execution department automatically.
That is, this order is set to Released automatically in the Customer Demand view of the Outbound Logistics Control work center and immediately appears in the Delivery Control view of the Outbound Logistics work center.
Rest Urgent, Normal and Low is only for reporting purpose.
Regards
Vignesh Ravichandran