This contrasts with the cleaning "horsepower" measurement of water-based cleaners and degreasers, which is the pH and which is the concentration of hydrogen ions in such a solution. Since solvent-based cleaners do not use water, they cannot have a pH. So they are commonly classified by their Kb value.
In terms of the test itself, the kauri-butanol value (Kb) of a chemical shows the maximum amount of the hydrocarbon that can be added to a solution of kauri resin (a thick, gum-like material) in butyl alcohol without causing cloudiness. Since kauri resin is readily soluble in butyl alcohol but not in most hydrocarbon solvents, the resin solution will tolerate only a certain amount of dilution. "Stronger" solvents such as benzene can be added in a greater amount (and thus have a higher Kb value) than "weaker" solvents like mineral spirits.