Different countries have their own tax laws. If you are a resident in one country and have income and gains from another, you may have to pay tax on the same income in both countries – or none of them. DTA – Double Taxation Agreements aim to avoid ‘double taxation’ or double non-taxation.
For example, an individual who is resident in Belgium, but has rental income from a property in another country, may have to pay tax on the rental income in both in Belgium and that other country.
To avoid double taxation (and of course a double non-taxation), many countries entered into Tax Treaties (DTA – Double Taxation Agreements), mainly based on international OECD-Standards.