The Siemens SENTRON 5SM9121-0KK is a 2-pole residual current unit (RC unit electronic) with an instantaneous trip characteristic, rated 16 A at 10 mA sensitivity on a 230/400 VAC 50 Hz supply. This is the core protection element for a Type AC RCD — it detects sinusoidal AC earth faults only, not pulsating DC or smooth DC faults, so it belongs in circuits where the load is purely resistive or conventional AC (heaters, lighting, general-purpose socket outlets away from VFDs or rectifiers).
Two-pole form factor means it switches both phase and neutral. The instantaneous design (no intentional delay) trips within fractions of a cycle at the 10 mA threshold — fast enough for supplementary protection against lethal shock in dry locations, but it will nuisance-trip on any upstream surge or capacitive inrush. Pair it with a matching SENTRON MCB (5SY series) in the same DIN-rail enclosure; the RC unit mounts ahead of or after the MCB depending on the wiring scheme, but the manufacturer's wiring diagram should be followed to keep the neutral conductor continuous through the unit.