The Siemens 3RH2122-2KB40 is a SIRIUS coupling relay sized S00, designed to switch auxiliary circuits — typically interposing between a 24 V DC PLC output and a contactor coil, or for signal isolation. It packs two instantaneous normally-open contacts, a suppressor diode across the coil, and mounts via screw and snap-on onto a 35 mm standard rail, with ±180° rotation and ±22.5° tilt allowance on vertical surfaces.
What the ratings mean for fit
The built-in suppressor diode means you don't add an external flyback diode across this relay when driving it from a transistor output. One less component, one less joint to check.