The unit is a currency-denominated balance or transaction moving through a bookkeeping close. It may begin as an invoice, bank balance, receivable, payable, intercompany amount, or other monetary item. The review question is not “which rate should every company use?” It is whether the evidence packet lets an authorized owner see the currency, amount, rate source, rate date, reporting period, resulting base-currency amount, and explanation of any difference.
That framing fits an offshore workflow because the person preparing a schedule may not own the functional-currency policy or reporting conclusion. The preparer can gather source records, apply an approved mechanical rule, compare the result with the ledger, and flag an exception. The client or qualified adviser remains responsible for policy choices and financial reporting conclusions.