OTM invoice line item cost reference

Before explaining the question, we wanted to Copy the shipment cost line level data which to the invoice line level when the invoice is created.

So, in this case each shipment cost line should be matched with the invoice line, and for this we've found a table INVOICE_LINEITEM_COST_REF.

But in this table, none of the 'Bills' are being inserted but only invoices are present. And also there was a case where few invoices were not present in the table.

Is there any way to know on what basis the data is inserted to the tables and also how to have a common reference using which the shipment cost lines can be matched with the invoice line?