The Siemens 5SM1646-6FD is a Type A residual current device (FI-Protector) rated 63A, 4-pole, with a rated residual operating current IΔn of 0.3A at 400V. The 0.3A trip threshold is a deliberate choice: high enough to avoid nuisance tripping on leakage from filtered drives and long cable runs, low enough to meet the 300 mA fire-protection requirement in many national installation codes.
The 63A rating at 400V AC is the continuous current the main contacts carry in a 3-phase 4-wire system (TN-S or TT). The 4TE width (roughly 70 mm on a standard DIN rail) occupies four module positions — plan the enclosure fill factor accordingly; a full 63A 4P RCD leaves room for the upstream MCB or switch-disconnector in a typical 12- or 18-way board. The 55 mm depth (T=55mm) clears most shallow enclosures but check gland-plate clearance if back-mounting.