When I use my debit card in other countries abroad the USA, what is the currency rates used?

The currency conversion rates used to determine the transaction amount in US dollars is either a rate selected by VISA from the range of rates available in wholesale currency markets for the applicable central processing date (which rate may vary from the rate VISA itself receives) or the government mandated rate in effect for the applicable central processing date. The conversion rate in effect on the processing date may differ from the rate in effect on the transaction date or posting date.