MARC 21 has the following formats: Bibliographic, Authority, Holdings, Classification, Community.

Libraries use MARC Bibliographic, authority, and Holdings formats. Chapter 17 includes examples using Bibliographic and Authority formats only.

Although RDA does not include guidance on record structures, it is common practice in MARC 21 (and in most US libraries) to use the bibliographic and authority record structures; however, these structures are not entity-based structures. For example, a MARC 21 bibliographic record may include elements and relationships that are defined under the original RDA entities work, expression, manifestation, and items, while also including relationships to other entities, such as person, corporate body, and works.

While the majority of library software available still uses variations of the MARC format, there is an increasing push to implement new XML-based formats due to their increased flexibility. The first author to write about the need to replace MARC was Roy Tennant in his 2002 Library Journal article "MARC Must Die." BIBFRAME is developed as one of the options to replace MARC, other XML options are also available. Even so, more than 20 years later, most libraries still use MARC, and a huge amount of legacy data is stored in MARC records.