A second pH reading your lab takes after adding a buffer solution — it estimates the soil's reserve acidity, which is what actually sets your lime rate.
Your regular soil pH (measured in water or salt) tells you how acidic the soil is right now. But two soils at the same pH can need very different amounts of lime, because one holds far more reserve (exchangeable) acidity than the other. The buffer pH measures that reserve: the lab adds a standardized buffer, re-reads the pH, and the drop tells it how much lime to recommend.
Different states use different buffers — SMP, Sikora, Mehlich, Adams-Evans, Woodruff — and each has its own calibration table. That's why a single national lime formula can be wrong: the right answer depends on which buffer your lab ran.
Related: Lime requirement · Adams-Evans buffer · Sikora buffer