Hi,
Please have a look at your purchase order or scheduling agreement.
In PO or SA, the user can add additional component over and above BOM.
Normally, when we create PO/SA for subcontract, the component is automatically flow from BOM but the user can add any additional components. When this is done, this will get consumed, whenever, we do MIGO or GRN while receiving at the plant.
R Rajagopalan