The Siemens 3RQ4055-1SM30 is an output optocoupler from the SIRIUS 3RQ4 family of coupling relays in a narrow 6.2 mm housing. It switches one normally-open contact rated for a thermal current of 6 A, driven by a 24 V DC control signal. The 6 A thermal current (Ith) is the continuous carry rating for resistive loads; for motor or inductive loads, derate per the AC/DC switching capacity in the datasheet. The 6.2 mm width means it fits tightly on a DIN rail — useful when panel space is at a premium and you need to isolate a PLC output from a downstream contactor or solenoid.
At 6.2 mm wide, it occupies minimal rail space — a row of these can pack tightly in a control panel alongside other SIRIUS components.