Automotive Zener TVS — what this part is
It is a bidirectional device with a single protection channel, clamping at 70.1V max from a 43.6V reverse standoff voltage. This is the qualification that separates it from a standard commercial TVS.
onsemi lists the P6SMB51CAT3 as Obsolete. SMB is a standard footprint; if you are looking for a production-active alternative, the SMB package is widely used across the P6SMB family and other TVS series from onsemi and Littelfuse.
The 43.6V reverse standoff voltage is the maximum DC voltage the protected line can carry without the TVS conducting. For a 48V nominal automotive bus (e.g., a 48V mild-hybrid rail or a 48V telecom feed), this part would start to clamp above that level — make sure your operating voltage stays below 43.6V. Clamping at 70.1V max with an 8.6A peak pulse current defines the voltage the downstream circuitry must survive during a transient. If your load's absolute-max rating is below 70V, this TVS may not protect it adequately. The 205 pF capacitance at 1 MHz matters for signal integrity on high-speed data lines (CAN FD, FlexRay, Ethernet). At 205 pF, it will load the bus; for lower-capacitance needs, look at a dedicated ESD diode with <10 pF.