What the clamping chain means for the protected rail
The 5KP12A-TP: When a transient pushes the line above 13.3 V minimum breakdown, the Zener Avalanche channel turns on and the clamp engages. The gap between 12 V and 19.9 V is the overhead budget the designer allocates to the transient amplitude before the clamp engages. The axial R-6 package is sized for this power density; through-hole leads conduct heat to the PCB copper and the surrounding air. The junction rating is the limiting plane, not the ambient.
Board fit and packaging
R-6 axial through-hole package — the standard lead-form for this power class, compatible with wave-solder assembly and hand-rework alike. CT serves prototyping and repair kits. Single unidirectional channel — polarity matters at layout. The cathode connects to the line to be protected; the anode grounds. A miswired unidirectional TVS will not clamp negative transients on that rail.