What it is and what the ratings mean
The Siemens 5SU1356-0KV40 is a combined residual-current device and miniature circuit breaker (RCBO) — a single module that does both jobs: 30 mA earth-leakage protection (Type AC, meaning it detects sinusoidal AC fault currents only, not pulsed DC) and overcurrent/short-circuit protection via the B-curve thermal-magnetic trip at 40 A rated current. The 6 kA breaking capacity (per IEC 61009-1) is sized for domestic and light commercial final circuits where the prospective fault current stays under that threshold; in a panel with higher SCCR upstream you'd need a current-limiting upstream MCB or fuse to keep the let-through within 6 kA.
Deployment context
Mounts on standard DIN rail in a distribution board or consumer unit. The 1+N-Pole form factor (switching the line, solid neutral) is the common European final-circuit layout. The 70 mm depth (D=70MM) fits standard enclosures; check gland-plate clearance if the board is shallow.