MARC21 is a standard format for encoding bibliographic and related information in machine-readable form. It is widely used by libraries to facilitate the sharing and exchange of data about books, journals, and other resources. The format allows for the consistent representation of information, making it easier for libraries to catalog and access materials. Developed by the Library of Congress, MARC21 includes various fields and subfields that describe different aspects of a resource, such as its title, author, publication date, and subject. This structured approach helps ensure that users can efficiently find and retrieve information from library catalogs and databases.