ISO 11649 RF creditor reference (SEPA structured reference) — validate or generate. ?ref=RF18539007547034 checks the ISO 7064 mod 97-10 checksum and format (RF + 2 check digits + up to 21 alphanumerics); ?generate=539007547034 computes the check digits and returns the full RF reference with print formatting. Complements iban-check and vat-check for SEPA invoicing agents. Purely local, instant.