What this XT1N frame breaker is
The ABB XT1NU3070AFF000XXX is a Tmax XT1N molded case circuit breaker in a 3-pole frame, rated 125 A with a thermal-magnetic fixed trip unit adjustable from 70 to 700 A. It comes with front flat terminals for panel wiring and uses a frame mount — no DIN-rail adapter needed, it bolts directly into the switchboard or enclosure bus structure.
What the 125 A rating means for your panel
The 125 A continuous rating is the breaker's thermal current (Ith) at 40 °C ambient — the current it carries without tripping in normal service. That sets the maximum load you can hang downstream: a 125 A feeder feeding a sub-panel, a large motor control center bucket, or a group of smaller branch circuits. The thermal-magnetic trip gives you an inverse-time thermal element for overload protection and a magnetic instantaneous element for short-circuit protection. The fixed trip range (70–700 A) means the magnetic pickup is factory-set; you adjust the thermal setting via the dial on the front face.