Sources
Primarily the ECB daily reference rates (~31 currencies, cross-rates computed via EUR). If the ECB feed is down, or you need a currency the ECB doesn't publish, open.er-api.com (~160 currencies) is used automatically. Rates are fetched once a day and cached for 24 hours; if a rate goes stale it is used with an age warning instead of blocking your import.
Manual rates always win
Under Settings you can set a fixed rate per currency — it is used regardless of what the sources say, until you remove it. If you want no external calls at all, the plugin works fully on manual rates alone.
The FX buffer
The buffer is a percentage per supplier, applied to the rate in the calculation: 2% → the rate × 1.02. It protects your margin against rate movements between price updates. Important: the buffer only affects the calculation cost and thus the price — the real cost in COGS is always buffer-free, so profit reports stay honest.
The sanity threshold
If a fetched rate moves more than the threshold (default 10%) against the last used rate, it is rejected — the previous rate is kept and a warning is shown (set a manual rate to override, or wait for the next fetch) — the guard against a faulty rate source repricing your whole catalogue. Rates are indicative reference rates for price calculation, not payment exchange rates.