UNESCO Nomenclature (more properly UNESCO nomenclature for fields of science and technology) is a system developed by UNESCO for classification of research papers and doctoral dissertations.
There are three versions of the system, offering different levels of refinement through 2-, 4-, and 6-digit codes.
[1] 11 Logic 12 Mathematics 21 Astronomy, Astrophysics 22 Physics 23 Chemistry 24 Life Sciences 25 Earth and space science 31 Agricultural Sciences 32 Medical Sciences 33 Technological Sciences 51 Anthropology 52 Demography 53 Economic Sciences 54 Geography 55 History 56 Juridical Science and Law 57 Linguistics 58 Pedagogy 59 Political Science 61 Psychology 62 Sciences of Arts and Letters 63 Sociology 71 Ethics 72 Philosophy