The Siemens 3RH1921-1KA02 is a SIRIUS auxiliary switch designed to snap onto S0-size contactor relays and power contactors. It adds one normally-open mirror contact per IEC 60947-4-1, rated for 10 A in AC-12 duty and carrying a 6 kV surge voltage rating. The 10 mm width means it tucks into a tight DIN-rail row without crowding adjacent devices.
Snap-on mounting to the contactor's front or side; a second unit can be mounted laterally, so you can stack two auxiliary blocks on one S0 base without extra brackets. Screw-type terminals accept 2x (0.5…1.5 mm²) solid or 2x (0.75…2.5 mm²) stranded, or 2x (20…16) and 2x (18…14) AWG. Strip length is standard for the terminal — no special tooling needed.