The ABB XT1HU3100AAA00NXXX is an XT1H molded case switch with a 125 A TMF (thermal-magnetic fixed) trip unit.
The XT1H frame mounts in standard panel or enclosure cutouts. TMF trip provides fixed thermal-magnetic protection — no interchangeable trip unit, so the 125 A rating is hard-set at the factory. For a panel OEM, that means the BOM line is locked to this ampere rating; a downstream load change requires a different order code rather than a trip-unit swap.