The "Country" custom field gives you the ability to choose a country out of a list which contains all of the countries of the world.
The characteristic of "Country" is the ability to choose if you want a list of Country Names or a list of Country Codes to choose from.
As you can see from the screenshot below, you need to choose the Type "ACF - Country" for the "Return Value" select box to appear.
Once you are in your Article's Edit screen, choose the "Fields" Tab to see the Country custom field as you can see in the screenshot below.
This selectbox contains all the countries of the world and you can fast search the one you want just as you type in.