The 1N6491US/TR is a 5.6 V Zener diode rated for 1.5 W continuous power dissipation. The 5 Ohm maximum impedance (Zzt) at the test current means the regulation voltage shifts by up to 5 mV per mA of load change — tight enough for many reference and bias applications, but not a precision bandgap substitute. The ±5% tolerance on Vz means a 5.6 V part can land anywhere from 5.32 V to 5.88 V; if your circuit needs a tighter window, you may need to sort or select a different grade.
Leakage, forward drop, and temperature range
Reverse leakage is specified at 500 nA maximum when the applied voltage is 2 V — well below the Zener knee, so this is the leakage you see in a circuit that back-biases the diode below its regulation voltage. The forward voltage (Vf) is 1.5 V maximum at 1 A forward current, meaning the diode can also serve as a moderate-current rectifier or flyback clamp if the application needs it, though the 1.5 W power limit still governs. The SQ-MELF (A-case) package is a hermetic glass body with metallized end caps — it handles thermal cycling and moisture better than plastic SMD packages, which is why you see it in avionics, downhole tools, and other high-reliability environments where a plastic SOD-123 or SOT-23 would risk cracking or delamination.