ISSN is a general subject classification, which divides the subject field into 22 categories, and each category contains several specific subclasses, each of which is identified by a unique code. ISSN is widely used in library, publishing industry and other fields, and it is one of the more common discipline classification code systems in the world.
LCC is a system for classifying and coding various books. It divides the subject field into 2 1 categories, and each category contains several specific subcategories, and each subcategory has a unique code identifier. LCC is mainly used in libraries and publishing industry, and it is a commonly used discipline classification code in the Library of Congress.
NSF is a system for classifying and coding scientific research fields. It divides the scientific research field into 24 categories, each category contains several specific subcategories, and each subcategory is identified by a unique code. NSF is mainly used in scientific research institutions and academic publishing houses, and is a commonly used discipline classification code of the National Science Foundation of the United States.
Generally speaking, subject classification codes can help academic circles and management departments to accurately classify and manage various subject areas, and help promote the development of academic exchanges and scientific research cooperation.