Purchase Order can be generated for the Service Order products on backed order.
In the navigation pane, go to Service then Service Orders
Create a service order with products on backed order
From the Service Order ribbon which is located on the top panel. Click on Purchase
You will be taken to the newly created purchase order screen
Review and update the Purchase Order as needed e.g. increase QTY to be purchased
Click on Complete to process the purchase order
Service Order shows the backed order part has been allocated and Percent Allocated is now 100%
Fill in the Repaired Description
Click on Complete to invoice the customer and complete the Service Order