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You measure the voltage Vmax at T max, Vmin at Tmin, take the mean as Tnom. So now you have a change of voltage, Vmax-Vmin over a temperature range Tmax-Tmin. So you divide by the range of the temperatures, Tmax- Tmin, so you get a certain voltage change for a 1 degree change in temp. You then divide by the Vnom. This is now a tiny number so you express it in parts per million.
It is the change in voltage (or the fraction of rated voltage) per degree change in temperature. The very basic definition is used in calculating in this formula. If the changes are small it is convenient to express in terms of ppm so the dividing factor of 10^6.