Panel fit and the C-curve decision
The 4.5 kA breaking capacity per EN 60898 (5 kA per IEC 60947-2) tells you it safely clears a fault up to that level without welding the contacts or cascading upstream. At 2 modular width units (36 mm wide, 90 mm tall, 76 mm deep, 70 mm installation depth), it snaps onto a standard DIN rail and leaves room for adjacent breakers in a crowded enclosure.
Thermal derating — the real-world current limit
At 40 °C it derates to 9.39 A; at 45 °C to 9 A; at 55 °C to 8.4 A. The breaker mounts in any position, which helps when the DIN rail is vertical or inverted in a tight gland-plate layout.