The A9N18371 is a 4-pole miniature circuit breaker from the Acti9 C120N series, rated 63 A at 30 °C with a C-curve (magnetic trip 5...10 x In). That C-curve means it handles moderate inrush — typical for general distribution loads like lighting, socket circuits, and light motor feeders — without nuisance tripping on startup. The 10 kA Icn at 400 V AC (EN/IEC 60898-1) and 6 kA Icu at 440 V AC (EN/IEC 60947-2) tell you the fault-clearing capacity under different standards: the Icn is the rated short-circuit capacity for MCB duty, while the Icu is the ultimate breaking capacity for industrial use.
Clip-on mounting on a 35 mm symmetrical DIN rail, 12 module pitches wide (108 mm). The 73 mm depth leaves clearance for standard enclosure depths — no need for a deep gland plate. Tunnel terminals accept 1...50 mm² rigid or 1.5...35 mm² flexible; torque to 3.5 N·m. IP20 finger-safe front, so it's fine inside a panel but not for wet locations.
Current-production status confirmed. For a BOM-freeze item, this is a low-risk line — no LTB clock ticking.