In = 125 A is the continuous current rating; the overload protection Ir is fixed at 125 A (not adjustable), and the short-circuit protection Ii is fixed at 10 × In (1250 A). That fixed-trip design simplifies ordering — no trip-unit adjustment to mis-set in the field — but it also means the breaker is a straight swap for a 125 A line, not a general-purpose adjustable frame.
Where it lives in the panel
This breaker mounts in a standard IEC distribution panel or motor control center. The nut keeper kit included with this -ZD00 variant is a detail worth noting: it keeps the terminal nuts captive during wiring, which saves time on a production line or when an OEM in China is assembling DC Power cabinets. No loose hardware to drop into the panel.