In this list of the world's sovereign states, exactly sixteen of the UN member states and observer states have a certain property P, and so does the disputed territory of Abkhazia.
- Exactly 2 of these countries are in South America.
- Exactly 5 are in Africa.
- Exactly 4 (not counting Abkhazia) used to be in the USSR.
This should be enough to ensure the puzzle has a unique solution; I can add more clues if necessary.
What is the property P? Which are the sixteen countries?