The AC-3 rating at 400 V delivers 5.5 kW, which covers typical three-phase induction motors up to that output; the AC-4 rating at 400 V is 8.5 A, relevant for reversing or inching duty where the contacts break full-load current.
The AC-4 rating of 8.5 A at 400 V is lower because that duty class involves breaking stalled-rotor current; if your application cycles the motor under load (plugging, inching), the AC-4 number governs the real contact life. The mechanical life of 30 million cycles means the actuator itself will outlast most electrical wear in normal use.
This contactor is built for a panel environment: IP20 on the front and at the terminals means finger-safe against accidental contact, but it needs an enclosure for washdown or outdoor use. The screw-type terminals on the main circuit accept 2x (0.5 to 1.5 mm²), 2x (0.75 to 2.5 mm²), or up to 2x (0.75 to 4 mm²) solid or stranded — typical for the control wiring in a motor starter cell. Side-by-side mounting is allowed with a 6 mm gap between units for heat dissipation, so you can pack several contactors on a rail without derating, provided the ambient stays within -25 to +60 °C. The altitude limit of 2000 m is standard for non-derated operation; above that, the air density drop reduces the arc-quenching capability.