Some countries, especially those who legally limit transactions in foreign currencies, have fixed their rates through legislation or a legislative body (perhaps a currency board or central bank). This rate is compulsory for transaction of certain types.
Such rates are useful for many purposes, but might reveal more about a country's macro-economic and political policies than its economic activity.
In such economies there will often be an unofficial rate determined by actual currency transactions.
Sometimes there are two or more rates. A main rate is often published as a 'principle' rate. Sometimes there will be secondary or tertiary exchange rates used for different purposes (for example tourist expenditure, direct investment from outside the country). For example, up until 2013, Cuba had two currencies, one for external dealings and the other for internal use. The 'convertible peso' was fixed to the dollar and the 'national peso' had an exchange rate to the 'convertible peso' set by the Cuban authorities.
In such cases interpretation of exchange rates for purposes of comparison need to take this into account.
Some countries maintain official exchange rates but allow them to change as market conditions change.
Whilst at the moment it is very difficult for a country to determine its exchange rate purely by command, over the time period covered in IMF databanks this has not always been the case. Many currencies have been subject to a number of different national exchange rate regimes - from state controlled official rates, through to 'floating' rates mainly determined by the foreign exchange market.
Check the nature of the exchange rates under comparison for the countries and time period being used, especially if the series include official rates.
