The point is that no tax is actually being paid in these cases. You have to specify them, but the amount of tax you were supposed to be paying (as if you bought the item in the Netherlands) has to be specified. You substract the same amount again in 5B. So net there is no change, but it is properly represented on the form.
I am not an expert on this tax topic, but I think Hans is explaining it incorrectly. Suppose you buy something from abroad and they reverse charge the VAT. The VAT you pay is zero. However, when reverse VAT (BTW verlegd) is conducted you are supposed to report these numbers. You are supposed to pay VAT based on this amount in the VAT category these items would be classified. Let’s say you are supposed to pay 21% VAT.
We have bought for 1000 euro’s, (“Grondslag”, 4a) the VAT amount would be 210 (“Omzetbelasting”, 4b) which is not payed to the supplier. As it is reverse VAT we are supposed to directly pay this to the Dutch tax adminstration. But, to keep the tax balance we add this amount from 5b (“voorbelasting”) which is the tax payed, so the net change is zero. This is discussed here (although in Dutch, Google Translate might help)
So looking at the numbers:
4a → 1000
4b —> 210
5a → 210 (summation of all categories 1 to 4)
5b —> 210 (tax payed in thay year, from tax calculation in manager)
5c—> 0 (in this example, with only the reverse VAT transaction)