A registration letter is a kind of proof of identity for your pleasure craft that proves that your craft is registered with the FPS Mobility and Transport. Then you also have the right to fly the Belgian flag.
A registration letter is mandatory both on inland waters and at sea (except on enclosed lakes).
If the vessel is longer than 20m and only sails on inland waterways, a union inland navigation certificate and a certificate of measurement are mandatory (these then replace the registration letter).
A registration letter is only required for pleasure craft from 2.5m to 24m in length (and also for personal water scooters). So not for kayaks, gondolas, canoes, pedal boats, sailboards, or other rigs smaller than 2.5 metres, nor for pleasure craft longer than 24 metres.
Furthermore, a registration letter is also not required for a pleasure craft used to transport more than 12 paying passengers or for sea angling with paying passengers on board. You do need a sea letter then.
More info: mobilit.belgium.be/nl/scheepvaart/pleziervaart/vaartuig/registratie (information in Dutch) or mobilit.belgium.be/fr/navigation/navigation-de-plaisance/navire/enregistrement (information in French)