subcontarcting process

hi

in subcontracting process we have finished material X and components A B and C

now we just provide C and vendor uses A and B from his own stock

now our requirement is we need to track the A and B component per PO

so how can i get this in BOM , is the Material provision indicator will be beneficial in this case

when i use this indicator as L , system wont show A and B component in PO only C is visible (as C is with out indicator L)

now can any one explain me how can i go with this scenario

regards

kunal