That 1.5 kW rating tells you the diode can absorb a substantial transient without failing — useful on 12V or 10V rails where the 10.2V standoff keeps the part out of conduction during normal operation, and the 11.4V minimum breakdown ensures it starts clamping before the downstream silicon sees damage. The 16.7V maximum clamp at rated Ipp is the voltage the protected circuit actually sees during a surge — budget that as the upper limit for any downstream device's absolute maximum rating, not a typical operating point.
Obsolete — sourcing reality for this BOM line
onsemi lists the 1N6273ARL4G as Obsolete. That means no ongoing production; the only available units are existing distributor or broker inventory. The DO-201AD axial package and through-hole mounting are common enough that a replacement search should focus on the electrical window: 10.2V standoff, 1500W pulse, unidirectional.
The axial leaded package solders into a through-hole board; the lead length and copper pad area on the PCB set the effective thermal resistance, not the package itself.