The 121 kA breaking capacity at 240 V tells you it can safely interrupt a massive fault without welding contacts or cascading upstream; at 415 V it still handles 75.6 kA, and at 690 V it holds 11.9 kA.
Fit and mounting
The 3VA1020-4ED36-0KA0 measures 130 mm high, 76.2 mm wide, and 70 mm deep.
Thermal derating and power loss
At 60 °C it derates to 19 A, and holds that through 70 °C. That's a clean thermal curve — no surprise drop at moderate cabinet temperatures. Maximum power loss is 12 W, which is modest for a 20 A MCCB; it won't drive a lot of heat into a densely packed panel.