Select "Default" in "Display in the country" if you want to display that specific country number to the rest of the places, besides what you have already selected.
For example, you want Australian to should see Australian numbers and US visitors to see US numbers. But, what about the rest of the countries.
Say, someone visits your website from Spain. What number should they see? In this scenario, you must select "Default" in "Display in the country" of the number that you want to display to the countries that are not specified in DNI.