Answer
When the documentation doesn't cover a specific report's parameter mapping, check the PL/SQL procedure that actually orders/generates that report — the report's own ordering procedure (e.g. something like Payment_Proposal_API.Order_Proposal_Advice_Report for Payment Advice) is where PDF_PARAMETER_n values actually get set, so reading that code tells you definitively which parameters a given report populates and which are simply left empty by design for that report.