Its TMF 100-1000 thermal-magnetic trip unit gives an adjustable continuous current range from 100 A up to 1000 A — so you set the actual protection point to match the downstream load, not the other way around. Three poles, line and load lugs standard, UL and CSA listed for North American panels.
What the Ratings Mean in the Panel
The 125 A frame is the mechanical envelope; the TMF trip unit is what decides when it opens. Thermal-magnetic means it handles two fault regimes: a thermal bimetal for sustained overloads (slow, heat-based) and a magnetic solenoid for short-circuit events (instantaneous). With the adjustable range of 100–1000 A, you can dial the continuous current rating to exactly what the branch circuit draws — a 150 A motor feeder gets set to 150 A, not the frame max. That adjustability is the difference between a breaker that's right for the load and one that's just big enough not to nuisance-trip.