*   Mandatory step for all implementations:
*   Copy the importing parameters to the changing parameters.
    MOVE-CORRESPONDING bil_doc TO bil_doc_res.
    MOVE-CORRESPONDING bil_doc_item TO bil_doc_item_res.
    MOVE-CORRESPONDING bil_doc_item_contr TO bil_doc_item_contr_res.

*   Copy solution order ID from item level to your custom field on billing document header level
    bil_doc_res-yy1_bus_solution_order_bdh = bil_doc_item-businesssolutionorder.

*Clear fields: Reference and Customer Reference
IF bil_doc_item-businesssolutionorder IS NOT INITIAL.
  CLEAR bil_doc_res-documentreferenceid.
  CLEAR bil_doc_res-purchaseorderbycustomer.
ENDIF.