Source Inspection, dock audit..etc, either by you or by your customer at the location point of the subcontracted activity.....
In other words..(am example)..your customer wants to see for himself that the subcontracted stuff meets HIS specifications/requirements before it ships to you and you do whatever you need to do to it.
The requirement is that you communicate this need to your subcontractor/supplier, allowing that option if it becomes necessary. This is usually covered by stating that if it becomes a requirement, then you will put that as a note on the PO...
hpoe that helps...
barb