Electric potential Vss (negative) in Diodes, Mosfets and BJTs
Take a 9V battery. Depending on which terminal you decide to set the multimeter reference with the black probe, you measure 9V or -9V. Take two batteries, put them in series, and now you have 18V battery. Or -18V battery. Or +/- 9V battery. It depends what you select as the 0V reference. Same thing with a 1 meter stick. If it is 1m above ground, halway buried into ground, or completely buried into ground, it is still 1 meter stick, you just have to choose the reference ground level.