What the 2-pole rating means for fit
The XT4NU3100FFF000XXX: Two poles means this breaker switches and protects both line conductors in a single-phase circuit, or two phases of a three-phase system. It's the configuration you'd reach for on a 208/240 V split-phase feed or a 480 V two-pole branch circuit where the neutral is solidly bonded and doesn't need switching. The XT4 frame is a standard panel-mount footprint — same cutout and bus-bar spacing as the rest of the XT4 family, so no rework if you're swapping in from a different rating within the series.