Package and hand-soldering fit
The 1N6011D comes in a DO-35 axial-lead package (DO-204AH), which is a through-hole part you can breadboard or solder into a perfboard without a hot-air station. The leads are sturdy enough for point-to-point wiring on a prototype.
Zener voltage and tolerance for precision
Nominal zener voltage is 30 V, and the ±1% tolerance is tighter than the typical 5% or 10% you see on general-purpose zeners — useful if you need a stable reference or a clamp voltage that doesn't drift much across units. Reverse leakage is only 100 nA at 23 V reverse bias, so it won't load down a high-impedance node when the zener isn't conducting.
Temperature range and power handling
Maximum power dissipation is 500 mW, typical for a small-signal zener in this package.