What you're actually mounting
The ABB 1SAZ421201R1003 sits in the TA-series solid state overload relay line, a motor-protection device that lives under a contactor on the din rail and trips the control circuit when running current exceeds the setpoint. It is listed under Circuit Breakers, Fuses & Protection → Relays → Solid State Overload Relays, which is the class most panel builders reach for when a thermal overload isn't precise enough for the motor it's protecting.
Panel-side fit
Solid-state electronics mean there are no heater elements to torque down; terminations are typically cage-clamp on the line/load side and a plug-in control wiring harness to the contactor, so a wireman can land this without disturbing the power wiring beyond the three phase leads.
Where this class of relay earns its keep
Solid state overload relays replace bimetallic thermal units where the application needs tighter trip-class accuracy, phase-loss sensitivity, or a wider current setting range than a heater strip can deliver — typically inverter-fed or reversing-duty motors where a thermal unit drifts with ambient. The TA-series form factor is the ABB-native fit for the corresponding AF-series contactors, so the wiring loom and mechanical link are matched by design rather than improvised in the field.