Zener TVS for 120V rail protection
Its reverse standoff voltage of 120V means it sits across the rail without conducting during normal operation, then avalanches when a surge pushes the line above the breakdown threshold.
The minimum breakdown voltage is 133V, so the diode starts conducting between 133V and the maximum clamping voltage of 214V at the peak pulse current of 2.8A. That 214V ceiling is what the downstream circuitry must survive — a 120V rail with a 214V clamp gives roughly 78% headroom above nominal, which is typical for this voltage class.
The copper pad area under the package sets the thermal resistance; a solid pour on the cathode side helps conduct surge heat into the board. Under a hot-air station the part lifts cleanly at 260°C with a gentle rock — the plastic body is rated for the full -65°C to 150°C junction range, so a single reflow cycle at standard lead-free profile won't stress it.