The number is, I believe, simply peak voltage. And personally I find the measure grossly misleading: an iPhone with an 80% battery has an almost useless runtime. It definitely isn’t 80% of the original runtime.
My iPhone could handle about 2 days with heavy usage. It was reduced to a day after 1 year. Now after 2 and half years, the battery dies in about 12 hours with the same usage. The battery health is 83% according to iOS. So it’s definitely not a good indicator for real life usage.
