Structural Parameters of the Economy: Evolution of Sectoral Models and the Role of Knowledge-Intensive Services

The article examines the evolution of approaches to the formation and interpretation of the sectoral structure of the economy under conditions of post-industrial, knowledge-intensive, and digital transformations. Despite the widespread use of the classical three-sector model, contemporary changes in the nature of economic activity – particularly the growing role of knowledge-intensive business services (KIBS), servitization processes, digitalization of production, and service integration – require a reassessment of traditional analytical frameworks for structural analysis. Methodologically, the study is based on a systemic and historical-logical approach, comparative analysis of international classification systems, synthesis of theoretical conceptions, and structural-logical generalization. The article systematizes scientific approaches to sectoral division of the economy, which makes it possible to identify key trends in the transformation from agrarian and industrial models toward knowledge-based and service-oriented economic structures. It is found that the classical three-sector model does not provide full adequacy in reflecting contemporary economic realities due to its focus on aggregated groups of activities, whereas extended approaches that account for knowledge-intensive, innovative, and digital components provide a more adequate interpretation of complex intersectoral interactions. Unresolved methodological issues related to the conceptualization and analytical status of the quaternary and quinary sectors are identified, substantiating the need for further theoretical development. The practical significance of the study lies in the formation of a theoretical basis for improving statistical and analytical classifications of economic activity, which may be useful for researchers, analysts, and policymakers. The findings can be applied in the development of economic policies aimed at supporting knowledge-intensive and innovative sectors and enhancing the integration of sectoral models into international classification frameworks.