Two poles mean it switches both line and neutral (or both phases on a split-phase system), so it's a full-isolation device — not just a single-pole with a switched neutral. That matters for any circuit where you need guaranteed disconnection of both conductors.
The 2-pole width is two modular units (18 mm per pole), so it takes 36 mm of rail space. No special busbar or comb — standard 10 mm pin pitch for the supply side.