The Siemens 3RV2021-0EA10 is a SIRIUS motor-protection circuit breaker in size S0, designed to protect motors against overload and short-circuit faults. That's a standard feature on SIRIUS breakers but worth confirming if you're swapping in from a brand that treats it as an option.
Clearance requirements: 50 mm upwards and downwards, 30 mm at the sides, zero forwards or backwards. That's typical for an S0 frame; the forward zero clearance is handy when panel depth is tight. Main circuit terminals accept 2x (1 to 2.5 mm²) or 2x (2.5 to 10 mm²) solid or stranded wire. Screw terminals with M4 thread for the main contacts, and the screwdriver shaft needs a 5 to 6 mm diameter Pozidriv size 2 tip.