It's the mounting frame for the ENW20H circuit breaker — you slide the breaker into this chassis, which stays wired to the busbars and control circuits. That drawout design means you can isolate and withdraw the breaker for maintenance or swap without disturbing the main power connections, a standard arrangement in switchgear lineups where uptime matters.
The chassis measures 445 mm deep by 400 mm wide by 440 mm high. That depth is the critical dimension for enclosure fit — verify the switchgear cubicle depth clears 445 mm plus the breaker projection and any rear-mounted bus connections. Width and height are standard for the EasyPact NW drawout platform, so retrofitting into an existing NW lineup should be a direct mechanical swap.