The Siemens 3RQ4018-2AE00 is a SIRIUS 3RQ4 coupling relay in the narrow 6.2 mm housing — an output coupler with an integrated relay that gives you one change-over contact rated for 6 A thermal current. Coil pulls in at 115 V AC/DC, so it works across both supply types without needing a separate rectifier.
At 6.2 mm per unit, this relay stacks tight on a DIN rail — you can pack several in the space a conventional screw-terminal relay would take. That matters when the enclosure fill factor is already tight and you're trying to avoid jumping to a bigger gland plate. The narrow SIRIUS 3RQ4 family is designed for high-density control cabinets where every millimeter of rail space counts.