The Work Order in the SERIAL file would be the work order that made the part (source of the product).  If you made a subassembly that was issued to a higher level work order, there is not a place in the SERIAL file for both the source and destination.
 
We are looking into a new program to track lot and serial controlled components mapped to a specific serialized assembly so if you have a work order to make 10 serialized assemblies and some components are lot or serial controlled there would be traceability of which component was put into which assembly.  Contact me if you are interested in more details or would like to make any suggestions as to desired functionality.