The Siemens 3RQ4118-2AF01 is a SIRIUS 3RQ4 output coupler relay in the narrow 6.2 mm housing. It switches one change-over contact at 230 V AC/DC with a thermal current rating of 6 A. The 6.2 mm width is the key space-saving feature — it packs a relay into a DIN-rail footprint that normally holds a terminal block, so you can add isolation or signal conditioning without expanding the enclosure. The hard gold-plated contacts are specified for switching low-level signals (milliamps, millivolts) where standard silver contacts would oxidize and fail to make reliable contact. This makes it a fit for PLC output isolation or sensor signal coupling, not for power loads.
The 6.2 mm width means you can fit roughly 16 of these per 100 mm of DIN rail, compared to 8 of a standard 12.5 mm relay. The plug-in relay base means the coil/contact module can be swapped without disturbing the wiring; the base stays on the rail. Coil draws 230 V AC/DC, so it works across both supply types without a bridge rectifier — one less thing to spec.