What this rating means on the rail
The Siemens 5SU1354-1KV40 is a combined RCD/MCB — an FI/LS protector — in a single 1+N-pole module, rated C-curve 40 A with 30 mA residual-current sensitivity and 10 kA breaking capacity. That 30 mA trip threshold means it catches the leakage level that protects against direct-contact shock in standard TN and TT systems, while the C-curve holds the magnetic release for motor and lighting inrush up to about 5–10× rated current.
The 70 mm depth (D=70MM) is the dimension from the DIN rail mounting surface to the front of the module — that's the space you need behind the cover or door.