The Type A fault-current detection catches pulsating DC residual currents from rectifiers or VFDs, which a plain Type AC unit would miss. Overvoltage category III means it's rated for fixed-installation downstream of the main panel.
Thermal derating — the number that actually decides your load
The 40 A rating is only valid at 30 °C ambient. At 40 °C it's 37.6 A; at 50 °C it drops to 34.8 A; at 60 °C it's 32.4 A; and at 70 °C it's 29.2 A. The mounting position is unrestricted, so you can stack these vertically in a crowded rail without derating further for orientation.
The 77 mm depth plus 70 mm installation depth.