The five MARC 21 communication formats, MARC 21 Format for Bibliographic Data, MARC 21 Format for Authority Data, MARC 21 Format for Holdings Data, MARC 21 Format for Classification Data, and MARC 21 Format for Community Information, are widely used standards for the representation and exchange of bibliographic, authority, holdings, classification, and community information data in machine-readable form.
A MARC record is composed of three elements: the record structure, the content designation, and the data content of the record. The record structure is an implementation of the international standard Format for Information Exchange (ISO 2709) and its American counterpart, Bibliographic Information Interchange (ANSI/NISO Z39.2). The content designation--the codes and conventions established explicitly to identify and further characterize the data elements within a record and to support the manipulation of that data--is defined by each of the MARC formats. The content of the data elements that comprise a MARC record is usually defined by standards outside the formats. Examples are the International Standard Bibliographic Description (ISBD), Anglo-American Cataloguing Rules, Library of Congress Subject Headings (LCSH), or other cataloging rules, subject thesauri, and classification schedules used by the organization that creates a record. The content of certain coded data elements is defined in the MARC formats (e.g., the Leader, field 007, field 008).
The MARC 21 Format for Bibliographic Data: Including Guidelines for Content Designation defines the codes and conventions (tags, indicators, subfield codes, and coded values that identify the data elements in MARC bibliographic records. This document is intended for the use of personnel involved in the creation and maintenance of bibliographic records, as well as those involved in the design and maintenance of systems for communication and processing of bibliographic records. This documentation is also available online, including a concise version and a simple field list at: www.loc.gov/marc/.