The article deals with the urgent task of improving the universal decimal classification (UDC) in connection with its numerous shortcomings. The examples given in the article show that the well-known rules for constructing classifications are violated in the UDC and the system principles for constructing classification models are not applied. These circumstances turn UDC indexing into a complex and extremely time-consuming creative, heuristic process, the result of which in many cases, in principle, cannot be correct. To solve the problem, a systematic approach to the construction of conceptual classification models (CCM) was applied, which allows representing a conceptual (conceptual) model of the form and content of knowledge, which will ensure the ease of its perception, memorization and use, as well as the uniqueness of solving the problem of choosing a document index on the content and form of information. The basic classification of knowledge and the classification of knowledge by content and form are presented, on the basis of which classifiers of specific subject areas can be developed.