A new category has been created for drones on the Belgian Civil register, and there are now five entries. Registration is mandatory for drones from 1 kg up to 150 kg, according to my source, and there’s an estimated 1000-2000 of those around in Belgium.
The weights must indicate MTOW, I should guess, and the text seems to indicate that no civilian drone may have moer than 150 kg.
(I know I should have consulted official sources instead of offering “seems”, “guess” &c., just can’t be bothered right now.)
The 150 kg limit comes from Annex II. And it’s operating mass. Heavier ones would fall under EASA.