When using Revolut card abroad it will choose the main currency based on your address. E.g. If you live in France then EUR is the default/main currency. So when travelling abroad and paying with Revolut it will try to debit from the EUR account and apply the exchange rate; although if the EUR account does not have enough money it will try to charge from the currency account that has the most money.
This behaviour is more or less ok. But it would be very useful for the user to be able to override this behaviour and select his main currency - even if it was for a while.
An example would be the case when the user is travelling abroad and paying in another non-supported currency (e.g. THB -Thai Baht) and wants the amount to be debited from another currency account (say CHF or GBP).