What does "Voltage Holding Characteristic Minimum" refer to for a supercap?
If you look at the bottom of the data sheet, the Voltage Holding Characteristic refers to the self-discharge behavior of the capacitor.  It is measured by charging the capacitor to 5 volts and then measuring the voltage after 24 hours has elapsed. For these particular capacitors, the voltage after 24 hours will be at least 4.2 volts.