The thermal-magnetic mechanism handles the overload (thermal) and short-circuit (magnetic) protection in one compact DIN-rail package.
The 1-pole width (roughly 18 mm per module) leaves space for adjacent breakers, contactors, or busbar feeders. The D-curve makes it a natural fit for a sub-panel feeding a small motor or a transformer — the magnetic trip won't nuisance-trip on the inrush, but still clears a hard fault fast enough to coordinate with an upstream S200 or S400 series main breaker.