What the three-voltage spec actually tells you
The 1.5SMC91A-Q: The 77.8V reverse standoff is the working rail the diode ignores — it sits off and draws only leakage current up to that level. That ~2.1:1 ratio between standoff and clamping is the let-through ceiling — tight enough for most 77V-rail protection targets in automotive and industrial DC distribution.
Surge energy and peak pulse power
The 61A peak pulse rating at the clamping voltage sets the surge current the diode survives without degradation per the AEC-Q101 test sequence; exceed that current and the clamping voltage rises above spec, so the board-level surge budget must stay within it.
Automotive qualification and temperature grade
The DO-214AB SMC package is the industry-standard TVS footprint; land pattern tolerance is forgiving and dual-source availability from multiple TVS manufacturers is strong for this case size.