In modern practice, reference moisture values are no longer based only on ppm in oil, but on the oil-paper equilibrium concept defined in IEC and IEEE guides. For paper insulation, a commonly accepted long-term target is below 2% moisture by weight for good reliability; 2-3% is considered aged but serviceable, and above 3-4% significantly accelerates aging and reduces mechanical strength. For oil, instead of fixed ppm, utilities increasingly use relative saturation or water activity (aw). Typical reference levels are aw < 0.2-0.3 for dry insulation, 0.3-0.5 as a warning zone, and > 0.6 where the risk of free water and rapid paper wetting becomes serious. These references allow consistent interpretation across temperatures and oil types, which was not possible with ppm alone.