What the ratings mean for your panel
The Schneider Electric A9F03440 is a 40 A, 4-pole miniature circuit-breaker from the Acti9 iC60N family, built for distribution boards and sub-panels where you need a thermal-magnetic trip in a compact DIN-rail package. The B-curve (magnetic trip at 4 x In ±20%) means it clears short-circuit faults fast enough for resistive and general-purpose loads, but won't nuisance-trip on moderate inrush from small motors or capacitive supplies — a common fit for lighting, socket-outlet, and control transformer circuits.
Panel integration notes
Snaps onto standard DIN rail (35 mm). The 4-pole body takes 8 x 9 mm pitches — 72 mm wide — so plan for that footprint in your enclosure layout. Terminals accept up to 35 mm² rigid or 25 mm² flexible on double terminals top and bottom; strip 14 mm and torque to 3.5 N·m. IP20 finger-safe protection, pollution degree 3 rated, so it's fine inside a closed panel but not for washdown zones.