It depends on the type of voltage AC or DC. In the case of DC there needs to be a reference level such as Earth ground for example. In my own case my DC level is les than 1 millivolt, the lower limit of my DC voltmeter. I checked a friend recently and his was -25 mv. I have heard of AC voltages in the range of 1-5 volts which is most likely RMS which is approximately .707 times the peak to peak for a given frequency. The purpose of the DC measurement might be to determine the amount of electrons available to prevent free radicals from causing inflammation followed by dietary disease. In the case of the AC data the purpose was to sell a product designed to lower the body voltage due to extraneous ac induced voltage.
When I was in school I used a voltmeter to measure my voltage, I can't remember exactly what it was but it was rli low like 0.1V but I'm not sure