The ABB S401M-C20 is a 1-pole miniature circuit breaker from the S400 family, rated 20 A with a Trip Curve C characteristic and a 10 kA interrupting capacity at 240 VAC.
Single-pole format means it occupies one module width — useful when you're packing multiple lighting or small-load circuits into a tight enclosure. The S400 series is ABB's compact MCB line, so the S401M-C20 slots alongside other S400 poles for multi-pole assemblies using the same busbar and accessory range.
How it compares to the 4-pole sibling
The closest peer is the S204-C32 — same Curve C, same 10 kA interrupting, but 4 poles and 32 A. If your panel was specced around the S204-C32, the S401M-C20 won't drop in without rewiring: pole count and ampacity are different.