The Type A residual-current element detects pulsating DC faults, not just pure AC sine-wave leakage, which covers modern electronics and variable-speed drives on the line.
The 300 mA sensitivity is a common choice for final subcircuits supplying socket outlets in commercial or industrial panels where nuisance tripping from inrush or leakage from multiple loads is a concern — it's less likely to drop out on cable capacitance than a 30 mA unit. The C-curve means the magnetic trip fires at 5 to 10 times rated current (160–320 A for this 32 A breaker), so it handles motor and transformer inrush without false trips.