The ABB XT4NU3100AFF000XXX is a 1-pole circuit breaker from the XT4N 250 TMF series, rated for 20 A and mounted directly to a wall or panel surface. Single-pole means it switches one phase, so it's a fit for 120/240 V split-phase or single-phase line-to-neutral circuits where you don't need multi-pole coordination.
Wall-mount form factor means it bolts directly to a backplate or enclosure wall — no DIN rail required. That's handy for retrofit work where the existing panel layout doesn't have rail space, or for standalone sub-panel protection near a motor or control transformer. The XT4N 250 TMF series uses a compact footprint; verify the drilling template against your gland plate before committing the panel layout.