Validate a structured payment reference and, when an IBAN is supplied, decide whether the two may legally travel together. USE WHEN: assembling a payment instruction from an invoice, a QR-bill or a remittance advice; whenever a Swiss IBAN and a reference appear together (the pairing rule is what most integrations get wrong); or when the user pastes an "RF..." string, a 27-digit number, a +++123/4567/89012+++ block, or asks whether a payment reference is correct. DO NOT USE to validate the IBAN itself — that is validate_iban. SCHEMES: RF Creditor Reference (ISO 11649, "SCOR" in Swiss Payment Standards, mod 97-10); Swiss QR reference ("QRR", 27 digits, modulo 10 recursive); Belgian OGM/VCS (12 digits, modulo 97, a remainder of 0 written 97); Finnish viitenumero (4-20 digits, weights 7-3-1 from the right). Norwegian KID and Swedish OCR are RECOGNISED but never judged: they answer valid: null with status unverifiable_without_creditor_config, because modulus type and length are configured per creditor account by the beneficiary bank and are not a property of the string. NEVER relay those to a user as "invalid" — say the check needs the creditor bank configuration. AMBIGUITY: only a leading "RF" and a 27-digit length pin a scheme down. A bare 12-digit string is both a Belgian OGM and a legal Finnish length, so the more specific reading is returned and the other appears in also_valid_as. Pass reference_type when you know the country. THE PAIRING RULE — the part no checksum library reproduces: pass an iban and you also get a pairing verdict. Per the Swiss Implementation Guidelines a QRR reference may ONLY be used with a QR-IBAN (institution identifier in the SIX range 30000-31999), and an ISO 11649 reference may NOT be used with one. Outside CH and LI, pairing is not_applicable — there is no QR-IBAN to pair against — and that does not affect the reference's own checksum verdict. IMPORTANT: valid and pairing are INDEPENDENT. A reference can be arithmetically valid and still illegal on that account. Read both, and relay source/as_of — they are what makes the verdict auditable. FREE: the checksums are published commodities. The paid surface is POST /v1/iban/validate, which returns this same pairing block with the full IBAN enrichment. COST: $0 per call, on every surface (free: 10 units/IP/day on this transport, one per call and one per IBAN in batch_validate_iban, or a free API key at POST https://api.ibanforge.com/v1/keys/generate for 200 REST calls/month).