The Siemens 5SY5140-6KK11 is a 1-pole miniature circuit breaker from the SENTRON series, rated 40 A with a B tripping characteristic and a 10 kA breaking capacity per EN 60898. It's a universal-current device — handles both AC and DC circuits, though DC polarity must be observed.
The 40 A rating is at 35 °C ambient; it derates to 37.56 A at 40 °C, 36.28 A at 45 °C, 34.96 A at 50 °C, 33.6 A at 55 °C, and 32.16 A at 60 °C. If your panel runs hot — common in a crowded enclosure — factor that curve in. Energy limitation class 3 keeps the let-through energy low enough to protect downstream wiring and loads.