If items on a sales order have a backordered quantity value other than zero, you can create a requisition for all of these items when editing the order.
When you click the Requisition button on the toolbar, the entire item list from the sales order is loaded into the Requisition work screen, with Required Quantities equal to the respective sales order Backordered quantities, ignoring any other quantity values from the inventory item.
From here, you can select just those items you want to requisition, and change quantities, costs or vendors for any item. Save all selections to the Requisitions list where the Required Qty is greater than zero. Or if you have user security rights, Process the items immediately into new Purchase or Production Orders, where each manufactured item becomes a new Production Order, and all other items are added to new Purchase Orders for each identified vendor.