5.6 V zener — the regulation point and the tolerance
The 1N6318US/TR: The nominal zener voltage is 5.6 V at the specified test current, with a ±5% tolerance — so the actual breakdown voltage for any given unit lands between 5.32 V and 5.88 V. That ±5% window is standard for a general-purpose zener and sets the regulation accuracy for the rail or reference it clamps.
500 mW power — the thermal ceiling in a hermetic package
Rated for 500 mW maximum power dissipation. The -65°C to 175°C junction temperature range covers military/aerospace and downhole environments where standard plastic-packaged zeners would fail.