This default value is taken from the customer register, but can be temporarily modified for a particular invoice. All currencies must be registered in the Currencies procedure in the Global Settings. You will find additional information regarding how to manage currencies in the system in the help chapter for that procedure.
When you change the currency, a question appears asking if the prices should be recalculated automatically. If you click Yes, the row prices will become recalculated according to the exchange rate in the Currencies procedures.