The kauri-butanol value ("Kb value") is an international, standardized measure of solvent power for a hydrocarbon solvent, and is governed by an ASTM standardized test, ASTM D1133.
[1] The result of this test is a scaleless index, usually referred to as the "Kb value".
A higher Kb value means the solvent is more aggressive or active in the ability to dissolve certain materials.
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 butanol (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.