Remittance information
The part of a payment that says what the money is for, so the recipient knows which invoice to mark as paid.
In ISO 20022 the element is RmtInf and comes in two forms. Unstructured (Ustrd) is free text, capped at 140 characters in SEPA. Structured (Strd) carries a machine-readable reference such as the RF creditor reference or a typed invoice number. Its predecessor in the Swift world was field 70 of the MT103, four lines of 35 characters into which everything from invoice numbers to "thanks for the loan" was poured without much order.
The difference decides how much manual work an accounts team does. A structured reference can be matched to the ledger automatically; free text means somebody guesses. Sweden solved this early with the OCR number, one reason Swedish bankgiro payments were unusually easy to reconcile long before anyone said ISO 20022.