On certain capacitors / supercapacitors you will see a Voltage Holing Characteristic Minimum Rating (V) as shown below in Kemet’s FC Series Datasheet.
This rating is calling out the self discharge characteristics of the capacitor in question. The exact testing specifications can verify, however in this datasheet they call out the JIS C 5160-1 test and clarify saying the capacitor is charged to 5.0VDC for 24hrs and then let stand for 24 hours in a specified condition. The results are >4.2V and this is the Voltage Holding Characteristic. Below is the test informaiton from the datasheet.