The 3P+N configuration gives you overcurrent protection on all three phases plus a switched neutral, common for three-phase final subcircuits in European-style panels.
The D-curve (10–20× In magnetic trip) is sized for loads with high inrush current — think motor starters, small transformers, or discharge lighting. A 25 A D-curve MCB will hold through the starting surge of a 7.5 kW motor on direct-on-line start, where a C-curve might nuisance-trip. The 15 kA Icu at 400 V is the key coordination number: it tells the upstream protection designer that this breaker can handle the fault energy at that point in the system. Mounts on standard DIN rail in the distribution board; the 3P+N width is 4 module spaces (72 mm).