The Siemens 5SU1656-6KV16 is an FI/LS combination unit — a residual current device (RCD) and miniature circuit breaker (MCB) in one 1+N-pole module. It's a Type A RCD with a 300 mA trip sensitivity paired with a B-curve 16 A overcurrent protector, rated for 6 kA breaking capacity. That 300 mA IΔN means it's sized for general socket-outlet and lighting circuits where nuisance tripping from small leakage currents (like electronics or surge suppressors) needs to be avoided, rather than for personal shock protection which demands 30 mA.
The D=70mm dimension is the unit's width in millimeters — 70 mm wide, which is 4 standard DIN-rail module widths (each 17.5 mm). That's the panel space it occupies in the distribution board.