What this part is
The ABB 1SAZ211201R1051 is a thermal overload relay, sitting in the solid-state overload class and catalogued under Circuit Breakers, Fuses & Protection / Relays / Solid State Overload Relays. The listing names it as a thermal overload relay by ASEA BROWN BOVERI. For a panel builder that means it lands in the motor-starter protection slot alongside the contactor, sized to trip the feeder on sustained overload rather than short-circuit current.