Once you've set custom FX rates, it helps to know exactly when Light uses them and when it falls back to market rates — especially if you transact in several currencies.
Where custom rates apply
Custom rates are used everywhere Light converts a foreign-currency amount: bills, sales invoices, payments, bank reconciliation, expenses, credit notes, purchase-order matching, journal entries, multi-currency bank postings, and the month-end FX revaluation.
A few areas always use market rates by design and are not affected by custom rates: the raw exchange-rate lookup, partner/external API rate feeds, approval-threshold checks, and MRR analytics. These are intentional — for example, MRR stays on market rates so your historical trend doesn't shift when a rate is added.
The one rule to know: all-or-nothing per conversion
When a conversion is between two non-EUR currencies (say USD → GBP) and you've set a custom rate for only one of them, Light uses market rates for that entire conversion — it never mixes one custom leg with one market leg.
What this means in practice: to be sure your custom rates are used everywhere, set a rate for every non-EUR currency you transact in, each month. If you set USD but not GBP, any USD↔GBP conversion falls back to market rates for the whole conversion — and the exchange-rates report flags it (see below).
Conversions between EUR and one other currency are always fine — EUR is the reference, so only that one currency needs a rate.
Seeing where a fallback happened
The exchange-rates report shows the custom rates in effect for the period, and flags the dates and currency pairs where conversions fell back to market rates because a currency was missing a rate (recorded once per currency pair per day, not per individual conversion). If a figure looks off, that's the first place to check — it usually means a currency is missing a rate for that month.