That 400 A holds from 40 °C through 50 °C ambient; at 55 °C it derates to 375 A, and at 70 °C it's 300 A — so if your panel runs hot, the real ampacity is lower than the nameplate. The interrupting ratings are substantial: 242 kA at 240 V, 187 kA at 415 V and 440 V, 121 kA at 500 V, and 40 kA at 690 V. For a main feeder or large motor branch, that means it clears high-fault events without upstream coordination headaches. It also has a communication function — the ETU550 supports data exchange over the SENTRON communication bus, so you can pull trip data and status into a PLC or BMS without adding a separate module. The max power loss is 98.5 W, which matters for enclosure thermal management if you're stacking multiple breakers.
The ETU550 trip unit is field-replaceable, and the auxiliary switches (2 HQ) are plug-in, so you can swap them without pulling the breaker.