Closed Sales order considered as demand in constrained supply plan

We have a sales order which has been shipped and is currently in "Closed" status at header and line level but is being considered as a demand in the Supplies and demand table with a Planned order pegged to it ( with status "Release" in supplies and demands table) in a constrained Supply plan. The Plan options setup is to have the "Past due Open Sales order" and "Past due Shipped Sales order" considered for 52 weeks. However, does this value consider a Sales order which is in "Closed" status. If so, what could be purpose of the a closed sales order being considered as a demand.