What it is and what it does
The Omron D4ER1A21N is a limit switch from the D4E-N series, fitted with a roller plunger actuator that requires 11.8 N to operate. The contact block gives you one normally-open and one normally-closed set (1NO+1NC), terminated via screw terminals. Rated insulation voltage is 250 VAC; for DC loads it switches up to 1 A at 30 VDC.
Where it goes and why it matters
IP67 rating means the switch body and the roller plunger seal against washdown and dust ingress — it lives on a machine frame or conveyor line where coolant, washdown, or outdoor exposure is routine. The screw terminals simplify field wiring; no special tooling needed. Roller plunger suits cam-actuated or slide-actuated positions where the actuator follows the motion rather than taking a direct head-on hit.