The Siemens 5SY6440-6 is a SENTRON 4-pole miniature circuit breaker rated 40 A at AC with a B-curve tripping characteristic. It breaks fault currents up to 6 kA per EN 60898 and 10 kA per IEC 60947-2 — the higher IEC figure is the one that governs SCCR coordination in an industrial panel, so that's your reference for selectivity studies. The 4-modular-width (4 MW) footprint and quick-assembly fastening system let it snap onto a DIN rail in any mounting position, which simplifies panel layout when you're packing four poles into a tight enclosure.
B-curve and derating in the panel
The B-curve (tripping between 3 and 5 times In) is the standard choice for resistive and general lighting loads where inrush is low — it will nuisance-trip on motor or transformer starts, so don't spec it for inductive branch circuits. The 40 A rating holds at 35 °C; at 40 °C ambient it derates to 38.4 A, at 50 °C to 36.76 A, and at 55 °C to 35.92 A. If your panel runs hot — say a packed enclosure with drives upstream — use the 50 °C figure for your load calculation, not the nameplate 40 A.