Clamping voltage — what survives the transient
The 1.5SMC6.8CA-M3/57T: With a 5.8 V standoff (typ) and a 6.45 V minimum breakdown, the device sits idle below that threshold in normal circuit operation and switches into avalanche conduction only when a transient exceeds the breakdown knee.
1500 W — the peak pulse power sizing anchor
1500 W (1.5 kW) peak pulse power is the product of the clamping voltage and the peak pulse current — it defines the energy the device can absorb without degradation during a single 10/1000 µs event. SMC-package TVS diodes at this power class rely on the PCB pad geometry and copper area around the cathode tab to conduct heat away from the junction; check the Vishay thermal resistance curve in the datasheet for your layout before committing to a derating margin.
Thermal envelope and the design headroom question
The -65 °C low end covers cold-start and outdoor equipment deployment without derating concerns; the limiting factor is always the high side when the device is absorbing transients in a thermally constrained enclosure.