The 45 mm width keeps the rail density high; you can pack several of these into a standard 600 mm panel section.
At 690 V the interrupting capacity drops to 2 kA — that's the limit for 690 V motor circuits; don't assume the 400 V rating carries over. The CLASS 10 trip means it opens within 10 seconds at 7.2× the thermal setting — standard for most general-purpose motor starting, but if you have a high-inertia load that needs a longer acceleration ramp, CLASS 10 may nuisance-trip and you'd step up to CLASS 20.
The 3RV2021-4DA40 uses ring cable lug connections on the main circuit — M4 screws. That means you need a crimp tool and ring terminals, not just ferrules. Clearance: 50 mm above and below, 30 mm to the side, zero forwards or backwards. That zero forwards clearance is handy for shallow enclosures — the breaker doesn't stick out past the DIN rail. Temperature compensation is built in for the -20 to +60 °C operating range, so the trip curve stays accurate across the panel's ambient swing.