The absolute thermodynamic temperature scale is an ideal scale based on thermodynamic laws, requiring theoretical devices like ideal gases for measurements. In contrast, the International Temperature Scale of 1990 (ITS-90) is a practical scale defined by real-world laboratory devices. ITS-90 serves as a calibration standard for the thermodynamic scale but is not a scale in itself. The key distinction lies in the practical applicability of ITS-90 versus the theoretical nature of the absolute scale. Understanding this difference is essential for accurate temperature measurement and calibration in scientific contexts.