In a small country like Belgium, it is not unusual for an individual to have interests outside the country. He may work abroad and receive foreign earnings. He may inherit from a non-resident or his children may live abroad. And he may buy property abroad.
Some tax rules have effect across borders. Belgian income tax is due on all earnings and income, including foreign source income. Inheritance tax is due when the deceased was domiciled in Belgium even on the property he has abroad. And that his heirs are living abroad does not change anything to that.
And when the other country also levels tax on the same income, or upon the death of a Belgian resident, there is double taxation. And there is a conflict between the Belgian and the foreign tax rules.
Belgium has its own rules (domestic rules) to prevent double taxation, but it has also signed double tax treaties with 96 states to prevent the double taxation of income, and it has signed a convention for the prevention of double taxation with France for inheritance tax and registration tax and Sweden for inheritance tax.