Rates use the USD inter-currency basis from a failover chain of free, no-key feeds (frankfurter → er-api → currency-api → last-good cache). Extreme rates pass through unclamped by design. A stale tag means every live feed was unreachable and the last-good cached rate was served. FX is for quoting only — money still books in each FTR's base currency at the bank's receipt-date rate.