Please answer a question about the following article about United States dollar:

The U.S. dollar is fiat money. It is the currency most used in international transactions and is the world's most dominant reserve currency. Several countries use it as their official currency, and in many others it is the de facto currency. Besides the United States, it is also used as the sole currency in two British Overseas Territories in the Caribbean: the British Virgin Islands and the Turks and Caicos islands. A few countries use only the U.S. Dollar for paper money, while the country mints its own coins, or also accepts U.S. coins that can be used as payment in U.S. dollars, such as the Susan B. Anthony dollar.

Although for some countries the dollar is not their official currency, how do they use the currency?
de facto currency