Using Shop Order Requisitions supply in Production Line Management

We are looking for ways to plan and control the production of high volume parts somewhere in the middle of a more complicated product. One way would be to use Production Line Management. But since the parts are serial tracked and we need as built structures, we cannot use production schedules. So generating shop order requisitions from the proposed schedule would be an option. However, MRP does not seem to see these as supply for its demand. I get a shop order requisition from MRP too. That does not happen with a production schedule. This seems inconsistent to me. What’s the idea?