Field help (BOMALTSCO) 

Several BOMs can be defined for a single product reference. They are identified by the use of the code, entered using two digits.
The BOM codes can be used to manage different structures of subcontracted products.
In this screen, only one subcontract BOM can be used; it is not mandatory.
If a default BOM code has been entered for the specified product and supplier, it is automatically suggested when accessing the field in entry mode.
After selecting the BOM code, the subcontract order is automatically populated with:

If the product-site is not listed by the supplier, a blocking or warning message can be displayed based on the value of the ITMBPSFCY - Supplier-site referencing parameter (ACH chapter, REF group).