It is used to protect personnel from hazardous machine motion by stopping the machine when an object (typically a hand or body part) breaks the beam array. This model is in the F3SJ series, which is Omron's standard-profile safety light curtain line. The series is designed for point-of-operation and access guarding on presses, robots, assembly lines, and packaging machinery.
The '20' in the order code indicates a 20 mm beam pitch — meaning the light curtain can detect objects as small as 20 mm in diameter. This is finger-protection class per IEC 61496-2, so it catches a hand reaching into the hazard zone before the fingers pass through. For applications where only a larger body part (arm or torso) needs detection, a 30 mm or 40 mm pitch model would suffice, but 20 mm is the standard for hand/finger safety.