The Siemens 5SM1646-0FD is a Type AC residual current protective device (RCD) — an FI-Protector — rated 63 A, 4-pole, with a 0.3 A rated residual operating current (IΔn) at 400 V. The 4TE (70 mm) width fits standard DIN-rail enclosures.
The 0.3 A IΔn provides personal protection against direct contact (30 mA is the threshold for supplementary protection; 300 mA is fire-prevention level; 0.3 A sits in between, typically used for equipment protection or where nuisance tripping from leakage is a concern). The 63 A rating handles the full load current of a sub-distribution board or a heavy branch circuit.