It's a current-production part, so you're not chasing an obsolete code. Rated for 277 VAC and 60 VDC, with a 6 kA interrupting rating on AC and 10 kA on DC. The 1-pole form factor keeps panel density tight; you can stack multiple units side by side in a single row.
The closest functional peer is the S201M-C40 — same 1-pole DIN-rail footprint, same 277 VAC / 60 VDC ratings, same 6/10 kA interrupting capacity. If your BOM was built around the C40 but the actual load is resistive (heaters, lighting, control power), the B20 drops in without rewiring and gives better nuisance-trip margin on those low-inrush circuits. The 20 A rating on this B20 versus 40 A on the C40 is the other obvious split — size the breaker to the load, not the curve.
Panel integration and field swap
The 1-pole width is roughly 17.5 mm — plan your enclosure fill factor accordingly. Terminal screws accept up to 25 mm² conductor; torque to 2.0 Nm. For a field swap, isolate the circuit, check the nameplate matches the PO (20 A, Curve B, 1-pole), and lever the old unit off the rail with a flathead screwdriver in the clip slot. No rewiring needed if the load hasn't changed.