That mechanical interlock makes it the right block for reversing a motor or switching between two power sources without a shoot-through. The coil is 240 VDC, so it's a DC-control part, not the more common 24 VDC or 240 VAC. Rated for AC-1 resistive loads at 20 A, it's sized for heater or lighting circuits, not inductive motor starts — the AC-3 rating would be lower, and this listing doesn't carry one. Breaking capacity is 110 A at 440 V, 80 A at 500 V, 70 A at 660–690 V, which tells you it can clear a fault on the line side without welding contacts.
Where it fits and how it mounts
The screw-clamp terminals accept 1.5–4 mm² solid or 0.75–4 mm² flexible wire, or 0.34–2.5 mm² with ferrules. Tightening torque is 1.3 N·m with a #2 Philips or a 6 mm flathead.