It is the mounting frame that accepts the plug-in breaker mechanism — the drawout design means the breaker unit slides in and out of the chassis without disturbing the power connections, which is the standard arrangement for switchgear where you need to isolate a feeder for maintenance or swap a unit under load.
If your switchgear room runs hotter, you will need to apply the manufacturer's derating curve, but for most industrial distribution boards this is a straight drop-in. The 4-pole configuration handles three-phase plus neutral, so it suits TN or TT earthing systems where the neutral needs to be switched.