Registration in the Commune

yes it's a legal obligation and no it's not a legal obligation to pick up a residents' card if you are EU but not Belgian, instead you get your legal proof of registration only on paper - but if and when you need to show ID, if you show this paper registration, sometimes you might get blank looks, as the vast majority of people do actually pay the small fee for the EU national residents' card. It's a legal obligation for all over 15 year olds to carry proof of registration with them, be it by card or on paper.