Its 75 ns reverse recovery time keeps the turn-off loss low enough that the heatsink stays manageable at switching frequencies above 50 kHz. The 1.3 V forward drop at 3 A is a typical ultrafast figure — expect it to run warmer than a Schottky at the same current, but the 600 V blocking voltage is what lets this part sit on a 400 V DC bus with margin.
STMicroelectronics lists the STTH4L06QRL as obsolete. If you have a production run that still calls out this exact suffix, the Tape & Reel variant is what the QRL designates — the base die is the same as the STTH4L06 in DO-15. For a new design or a long-run BOM, the sensible move is to qualify a current-production fast recovery diode in the same voltage and current class; the 600 V, 4 A, 75 ns trr combination is a common enough target that several manufacturers offer a direct functional equivalent in a DO-15 or SMD footprint.
Package and mounting — through-hole axial
The DO-204AC (DO-15) axial package is a through-hole form factor that suits point-to-point wiring on perforated board or traditional PCB through-hole assembly.