What you're sourcing
The ABB 1SAZ411201R1001 is a thermal overload relay in the thermal overload heaters for starters class — a heater module that bolts to the contactor to give the starter its motor-overload trip curve.
Where it sits in the panel
In the starter stack, this class of heater rides under the contactor: line side from the branch breaker, load side out to the motor, and the aux/N.C. contact feeding the contactor coil so an overload event drops the coil and stops the motor. Sourcing the heater by its own order code is the normal practice when only the heater has failed or when the original-fit heater range needs to be repeated on a rebuild.