That C-curve means it's designed for moderate inrush loads — think control transformers, small contactor coils, or lighting circuits — where the start-up current is a few times the rated value but not the high surge of a motor start.
At 40 °C ambient the breaker derates to 0.28 A, and at 60 °C it's down to 0.24 A. The 0.3 A nominal rating is at 35 °C; above that, the load must be reduced or the breaker will nuisance-trip on a continuous current that's still below the nameplate. That's the number to use for sizing, not the 35 °C figure.
Width is 18 mm — one modular unit on a DIN rail. Mounting position any, fastening via the quick-assembly system (snap-on). That's a standard distribution board environment — no special sealing needed.
Rated to IEC/EN 60898-1 and IEC/EN 60947-2, plus UL1077. EN 60898 covers domestic/light commercial; IEC 60947-2 is the industrial standard with 35 kA breaking capacity. Halogen-free and silicon-free. Lifecycle stage is current.