It protects a 1P+N final subcircuit against both overload/short-circuit (B-curve magnetic trip at 20 A) and earth leakage (Type A, sensitive to pulsating DC fault currents).
Thermal derating — the real-world current limit
The 20 A rating holds only at 30 °C ambient. In a crowded enclosure at 40 °C the continuous current drops to 18.8 A; at 50 °C it's 17.4 A; at 55 °C (the upper operating limit) it's 16.2 A. If your panel ambient runs 60 °C the part still carries 16.2 A, and at 70 °C it's 14.6 A. That derating curve is the number that governs the real load — size the downstream circuit for the temperature you actually see inside the board, not the label on the front.
The 5SU1354-6FM20 snaps onto DIN rail, occupies 2 width units (36 mm), and accepts conductors from 0.75 mm² up to 35 mm² — solid or stranded. Mounting position is unrestricted, and the supply can enter from either top or bottom. The IP20 rating applies only when the distribution board is installed and conductors are connected — treat it as a panel-internal part, not a standalone surface-mount device.