Magnesium oxide was the highest reflectance across the visible spectrum at the time of the development of brightness and opacity. Therefore, the reflectance of magnesium oxide was set as the 100% point on the brightness scale in the 1930's. Likewise, magnesium oxide was going to be used as the white backing for the measurement of opacity. The problem was that magnesium oxide was very unstable (it yellowed quickly when exposed to air) and it was a chalky material which would be affected when coming into intimate contact with paper. The solution was to place a piece of glass between the magnesium oxide and the paper. Magnesium oxide was replace with magnesium carbonate which was more stable. These two changes led to a white body with a reflectance that was 89% of the reflectance of magnesium oxide.
So in measuring TAPPI T 425 Opacity (contrast ratio), there is a comparison of the reflectance of a single sheet is measured with a black body backing (R0) and then the reflectance of the same single sheet is measured with a white body having a reflectance of 89% (R0.89).
This differs from printing opacity which compares the reflectance of a single sheet with a black backing (R0) to the reflectance of the same single sheet with an infinite pad backing (R∞). That being said, the basis for the opacity scale is still related to the reflectance of magnesium oxide as the white body backing.