The 1.5SMC15A-E3/9AT: Breakdown occurs at a minimum of 14.3 V, so the diode stays off below that threshold and only conducts when the surge pushes the line above it. The 12.8 V reverse standoff is the maximum continuous DC voltage the part can tolerate without conducting — a 12 V rail with 10 % tolerance sits safely under that.
DO-214AB (SMC) is the standard surface-mount TVS footprint — the same package as the SMCJ series. The copper pad area under the cathode tab sets the thermal resistance; a larger pad pulls heat from the die during repetitive surge events. The -65 °C to 150 °C junction temperature range covers automotive under-hood and industrial outdoor environments — no derating needed at 85 °C ambient for single-pulse events, though repetitive surges require thermal calculation.