The Siemens 3RV6021-4BA10 is a SIRIUS motor protection circuit breaker, size S0, rated for 20 A operational current at 400 V AC-3 duty. That 20 A is the motor-running current it protects — not the inrush, not the short-circuit rating, but the continuous full-load current the overload element sees. The thermal overload release (Class 10) means it trips fast enough to protect a standard squirrel-cage motor during a stall or jam, before the windings cook. It carries a 690 V rated insulation voltage and a 6 kV surge voltage rating, so it's comfortable on 400 V line-to-line systems with decent headroom for transient spikes. The magnetic short-circuit trip handles the instantaneous fault clearing; the thermal element handles the overload curve. Three poles, screw-type terminals top and bottom.
Any mounting position is allowed. If you're packing this into a gland plate or a tight sub-panel, that 45 mm width per pole is the fill factor to watch.
Relative humidity tolerance 10 to 95 %. Shock resistance rated at 25 g / 11 ms per IEC 60068-2-27. Installation altitude limited to 2000 m above sea level. RoHS compliance date is 05/01/2012 — the part meets the RoHS directive from that date forward.