The Siemens 5SL3220-6 is a SENTRON miniature circuit breaker — a 2-pole, B-curve device rated 20 A with a 4.5 kA breaking capacity per both EN 60898 and IEC 60947-2. It's designed for residential and infrastructure branch-circuit protection where the fault level stays under that 4.5 kA ceiling at 400 V AC. If you're feeding motor or transformer circuits that see a higher magnetizing surge, you'd want a C or D curve instead; this one is sized for the less aggressive start.
Thermal derating — the real-world current you can actually run
At 40 °C it derates to 18.79 A; at 45 °C to 18.4 A; at 55 °C to 16.8 A. If your panel runs warm — and most do, especially with adjacent breakers — that 20 A nameplate isn't what you can continuously load. Size your downstream circuit for the derated figure at your actual enclosure ambient.