What this TVS protects and where it goes
The STMicroelectronics SM6T250CAY is a bidirectional Zener TVS diode rated for 600 W peak pulse power (10/1000 µs waveform) with a minimum breakdown voltage of 237 V and a clamping voltage of 344 V at 1.75 A.
Breakdown voltage — the protection threshold
The 237 V minimum breakdown sets the voltage at which the device starts conducting avalanche current. For a 213 V reverse standoff, the TVS remains transparent during normal operation and only clamps when a transient pushes the line above that threshold. If your rail sees steady-state ripple near 213 V, you need headroom: the 237 V minimum ensures the TVS does not clip during normal swings.
Automotive-grade qualification
That is the difference between the SM6T250CAY and the commercial SM6T250A: the CAY variant carries the automotive pedigree, so it is the one to specify for production vehicle ECUs, battery management systems, and any board that must survive the thermal and electrical environment of a vehicle.
No last-time-buy notice, no end-of-life announcement.