What the ratings mean for fit
The Siemens 5SU1656-1KV20 is an FI/LS combination unit — an RCD and MCB in one 70mm-wide module. The Type AC residual-current detection (300mA IΔn) handles sinusoidal AC fault currents to ground; it's the standard choice for general-purpose socket-outlet circuits where no DC or pulsed DC fault content is expected. The 6kA breaking capacity (IEC 60898) covers most domestic and light-commercial distribution boards fed from a transformer up to that prospective fault level. The C-curve 20A thermal-magnetic trip means it holds inrush from motorized loads (pumps, compressors) up to about 10x rated current, then trips magnetically above that.
Where it goes in the panel
The 70mm width (4 module spaces on a standard DIN rail) means it occupies the same footprint as a separate 1+N RCBO of comparable rating. In a distribution board, it snaps onto the DIN rail and connects via the busbar system. The 1+N-pole configuration switches both line and neutral, with the neutral pole switching but not protected against overcurrent — standard for single-phase final circuits where neutral isolation is required.