Avoiding double taxation
Because you reside in the Netherlands, you have to declare all of your income, even if the tax levy on that income has been designated to Belgium. However, you do not pay tax on that income twice.
We first calculate the tax on your entire income. After that, we calculate the tax that you must pay on the income that is also taxed in Belgium. For that amount, you will get a relief from the tax to be paid by you. This is called double tax relief (on the tax return notice this relief is called 'relief for taxation elsewhere').