Rated operational voltage is 400/690 VAC and 375 VDC, with a 50 kA interrupting capacity at 400 VAC — that's the fault current it can safely clear without welding contacts or venting arc gas into the panel. Snaps onto a DIN rail, so it integrates into any standard distribution board or sub-panel without extra bracketry.
The S803S-D63 shares the same 3-pole, Curve D, DIN-rail footprint and 50 kA interrupting rating — the only difference is the 63 A continuous rating versus 40 A. If your load draw is under 40 A, the D40 is the right call; if it's between 40 and 63 A, step up to the D63.