Currency conversion factors can replace official exchange rates, e.g. when the latter are considered to be especially unrepresentative of rates effectively applied to international transactions.
The creation of the Euro is a prominent example for the use of conversion rates: the irrevocable fixing of conversion rates came into force January 1, 1999. Once defined by the Council of the European Union, they replaced the former floating exchange rates between the participating national currencies.
AMECO Annual macro-economic database provides all conversion rates between Euro and former Euro-zone national currencies.