Four poles handle three-phase plus neutral or a second protected phase, depending on your distribution scheme.
Breaking capacity is the headline number here — 330 kA at 240 V, 242 kA at 415 V and 440 V, 187 kA at 500 V, and 3 kA at 690 V. The drop to 3 kA at 690 V means it's not your go-to for 690 V distribution unless the fault current is very low — check your SCCR study before specifying.
The ETU320 is an electronic trip unit — that means you get adjustable long-time pickup and delay, short-time pickup and delay, and instantaneous settings, plus ground-fault capability if the breaker is fitted with the right accessory (this one ships without ground-fault monitoring, per the evidence).
Four-pole width at 140 mm means it takes up the same rail space as two standard single-pole breakers side by side. IP40 on the front is typical for indoor panel mounting; no special sealing needed for dry environments.