Nowadays, having either professional activities or investments abroad is not rare. This can however cause some challenges from a tax point of view, especially for expats. And mistakes can be very costly. Here are some basic points of international taxation and how it fits in the tax system in Belgium :

The notion of "Realm inhabitant" (rather odd and awkward translation of "Habitant du Royaume/Rijksinwoner") is central in Belgian tax law. Anyone who falls under this definition will be subject to personal income tax in Belgium.
Whether someone is or is not a "Realm inhabitant" has to be determined based on each situation individually. Several factors will be taken into account : being registered as a resident in Belgium, owning a house or flat in Belgium, Having children going to school in Belgium, etc.

If you are subject to personal income tax in Belgium, it means that in principle your worldwide income will be taxable there (unlike non-residents who are only taxed on some specific income that have their source in Belgium eg. a US resident who owns a flat for rent in Belgium).

This is the basic principle, although there are exceptions. Typically, to avoid problems of double taxation, there are bilateral agreements between several countries that determine for certain types of income, which country has the power to levy taxes. This has to be analysed on a case-by-case basis.