That AC-3 rating means it's built for switching motor loads — the making and breaking of currents typical during induction motor starts and stops, where the inrush can hit 6-8x the full-load amp draw. This isn't a resistive-load relay; it's sized for the contact wear a motor presents, so it holds up in a starter or coupling circuit.
What the rating means in the panel
The 3 kW / 400 V AC-3 figure governs the motor size this relay can handle directly. On a 400 V three-phase supply, that's roughly a 4 hp motor. The coupling relay sits between the control circuit and the motor contactor coil — it isolates the PLC or safety relay output from the coil draw, which is typically higher than a standard 24 VDC digital output can sink. If you're wiring this into a panel, the AC-3 rating tells you the main contacts are rated for the motor's starting current, not just the sealed current, so it won't weld shut on the first start.