At 0.3 A it's a low-current branch, often used for control transformers, contactor coils, or PLC power supplies inside the panel.
Panel fit and derating
Width is 18 mm — one modular unit on a DIN rail. Thermal derating is published: 0.3 A at 30 °C, 0.28 A at 40 °C, 0.27 A at 45 °C, 0.25 A at 55 °C. In a crowded enclosure where ambient hits 40 °C, figure 0.28 A as the continuous rating — the breaker won't nuisance-trip on a 0.3 A load at that temperature. That lets you add remote status or trip indication without replacing the breaker.