You can add additional (for example customer-specific) items to a billing document (see Enhancements of the Billing Document).

If the cardinality of the information does not exactly correspond to the billing document items, you can store customer-specific information in the billing document additional items or link customer-specific tables and data records with the billing document.

You can fill the additional items in BAPI BAPI_ISTBILLDOC_CREATEMULTIPLE when the billing documents are transferred (see Transferring Billing Documents in the documentation for invoicing).