Temperature derating — the real current rating
The 20 A rating is at 35 °C ambient. In a warm enclosure — say 50 °C — it derates to 17.7 A. At 60 °C it's 16.42 A. If you're sizing this for a hot railway cabinet near traction equipment, use the 50 °C or 55 °C column, not the headline number. The breaker also has a humidity limit: max 95% RH up to 55 °C, dropping to 35% RH at 75 °C.
It's 36 mm wide (2 modular width units), 76 mm deep, and installs in any orientation. Both top and bottom accept combined terminals, and it's IP20 with conductors connected — fine for a closed panel, not for wet locations.