The Square D MGP26400 is a PowerPacT M electronic-trip circuit breaker in the MJ unit-mount form factor. Rated 400 A continuous, 2-pole, with an interrupting capacity of 18 kA at 600 VAC. Lugs come off the end for line-side or load-side connection in a panelboard or switchboard enclosure.
The 18 kA interrupting rating at 600 VAC is the short-circuit current rating for the breaker itself. Upstream protection must be coordinated so the available fault current does not exceed this value. That means you can dial in the trip settings to coordinate with downstream breakers and avoid nuisance tripping on motor inrush or transformer energization. The unit-mount (MJ) form factor bolts directly into a panelboard or switchboard bus, not a DIN rail — plan for a dedicated enclosure cutout and bus connection.