What it is and where it fits
The 3RH1921-2DA11-ZX95 is a lateral-mounted auxiliary switch block with one normally-open and one normally-closed contact, built to DIN EN 50012. It uses cage-clamp terminals and is sized for Siemens motor contactors in the S0 through S12 frame range. The 10 mm width means it snaps onto the side of the contactor without eating extra DIN-rail space — useful when you need feedback or interlocking on a contactor that's already packed into a panel.