The fixed 125 A rating means you don't adjust it; you size the feeder for that number and the breaker holds it.
Breaking capacity — what the curve tells you
This MCCB delivers 121 kA interrupting capacity at 240 V, dropping to 75.6 kA at 415 V, 52.5 kA at 440 V, 17 kA at 500 V, and 11.9 kA at 690 V. That's a steep derating as voltage climbs — common for a thermal-magnetic breaker. At 240 V it handles high fault currents typical of North American 120/240 V split-phase services; at 400 V class it still clears 75.6 kA, which covers most industrial distribution transformers up to about 2 MVA.
Thermal derating — don't ignore the ambient curve
At 55 °C it derates to 243 A, then 237 A at 60 °C, 230 A at 65 °C, and 223 A at 70 °C. If the panel sits in a hot mezzanine or near a furnace line, that 223 A at 70 °C is your real limit — not the nameplate 125 A.
Dimensions are 140 mm wide, 158 mm high, 70 mm deep. The 4-pole body is wider than a 3-pole — check your DIN-rail or mounting-plate space before you cut the backpanel.