Its 6.4V minimum breakdown voltage and 9.6V maximum clamping at 156.2A peak pulse current (10/1000µs waveform) define the protection envelope for sensitive 5V logic, sensor, or communication lines.
The 5V reverse standoff voltage is the maximum DC or AC peak the line can carry without the diode conducting — stay below this for normal operation. The 6.4V minimum breakdown is where clamping begins; the 9.6V maximum clamp at 156.2A is the voltage the protected circuit must survive during the surge.
No direct second-source or replacement is cited by Microchip; the SMCJ family is widely cross-shopped with Littelfuse and Vishay equivalents in the same DO-214AB footprint — confirm the breakdown and clamping thresholds match your design margin.