The Siemens SENTRON 3VA9573-0JB23 is a TA3.1 circular conductor terminal — a wire connector accessory for the 3VA55, 3VA65, and 3VA66 molded-case circuit breaker families. It ships as a set of three units, each accepting two cables, so you can land both line and load conductors on the same breaker pole without piggyback lugs.
The terminal accepts stranded copper or aluminum from 4/0 AWG up to 600 kcmil — that's a wide range covering typical feeder and large branch-circuit conductors. Finely stranded copper (Class B or similar) is also supported, same 4/0 to 600 kcmil range. The stripped length is 1.0 inch, so when you prep the wire, that's the bare conductor you need exposed before insertion. Torque spec is 42.4 Nm for both stranded and finely stranded sizes, which is a firm pull — use a calibrated torque wrench, not a guess.