That means onsemi has stopped manufacturing this exact order code. If you have a board in production or a repair that calls it out, you need to secure remaining stock or identify a drop-in replacement before your inventory runs dry.
The 28.2V reverse standoff voltage means it sits across a 24V rail without conducting in normal operation. When a surge hits, breakdown starts at 31.4V minimum and the clamp voltage peaks at 45.7V at 33A. That 45.7V ceiling is what the downstream circuitry must survive.
Housed in a DO-201AD axial leaded package, through-hole mount.