Abstract
The definition of a system of units is analyzed here as a structural problem, for which the physical theory provides the necessary conditions of realization but remains as backgrounder. The structurally more sophisticated option is provided by the so-called Global Constant Definitions, discussed in this paper through a conceptual roadmap intended to separate the components of the definitions based on theoretical foundation from those related to conventional features and, at the same time, to ensure the structural stability of the system and its partial modularity and therefore flexibility. The critical case of the so-called 'New SI' is used as an illustration example.