StrdCdtrRef is the camt.053 carrier field at <TxDtls>/<RmtInf>/<Strd>/<CdtrRefInf>/<Ref>. It is where banks echo structured remittance information — typically a SCOR (ISO 11649) reference or a Swiss QRR — back to the customer.
When a customer issues invoices with structured references, StrdCdtrRef is the highest-quality reconciliation carrier: it was set at issue time, it survives the rail unchanged, and it is meant to be the structured machine-readable handle. iso-compliant promotes StrdCdtrRef candidates to high confidence in apps/api/src/lib/recon-key-extract.ts when the value is a valid SCOR mod-97 or a valid QRR mod-10.
The carrier is rarely populated on legacy / unstructured flows. When a customer's invoice references are free-text strings instead of structured references, the same data tends to arrive in <Ustrd> (free-text remittance) and the matching engine has to regex-parse it.