Maximum take-off mass (MTOM) is the total mass of an unmanned aircraft — including its battery, camera and any attached payload — ready for flight. It is one of the defining parameters across the EASA open category.
MTOM decides which class a drone falls into (for example, under 250 g is C0) and which subcategory (A1, A2 or A3) it may be flown in. The manufacturer's MTOM figure therefore directly drives whether a certificate is needed and how close to people you may fly.