This field is specifically denotes the qty scrapped at supplier while working with the external service flow.
The field help also describe a bit on it.
“The quantity scrapped at the supplier, if the purchase order line is an external service order and parts were scrapped at the supplier because they could not be repaired.”
There is no automatic replication of qty values in the process, where as there is an Action button in Purchase Order lines with Order Code 6 (External Service Order flow) for the PO user to report the qty scrapped at supplier using the exact Lot/Serial involved.
UPDATE - after a review, it appears the only possible solution (we use order code 1) would be to change the qty to inspect at receipt and perform the scrapping there. It’s not the cleanest solution, but this does allow us to perform a scrapping function.