The 20 A rating suits it for lighting, receptacle, or small motor branch circuits in a 230/400 VAC distribution board. Keep an eye on the thermal derating curve if you're packing multiple breakers side-by-side in a sealed enclosure — the 20 A is at 30 °C ambient; above that you'll need to reduce the load.
The S273-C20 shares the same physical footprint and DIN-rail mount as the S273-K6A (6 A) and S273-K40A (40 A) siblings. The C-curve trip characteristic (5–10× In) on the C20 is the same magnetic trip profile used across the S273-C range — no difference in the mechanism, just the thermal element. If you're swapping from a K6A to a C20, the rail clip and wiring terminals are identical; the only change is the overcurrent pickup point.