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MODS User Guidelines Version 3

Element Attributes Subelements
<identifier> type


Contains a unique standard number or code that distinctively identifies a resource.


Guidelines for Use

<identifier> includes manifestation, expression, and work level identifiers.

Repeat the <identifier> element for each applicable identifier recorded, including invalid and canceled identifiers.


Specific DLF Aquifer Guidelines

The DLF/Aquifer Implementation Guidelines for Shareable MODS Records recommend the use of at least one <identifier> element with the type attribute containing appropriate values in each MODS record.

The <identifier> element is recommended in these guidelines and refers to the digital resource being described or to its analog equivalent. In the shared metadata context, the <identifier> should identify the object universally; local call numbers or other identifiers, therefore, are not generally suitable. However, if a local identifier is used, it should be indicated as such using the type attribute.

The guidelines recommend that <identifier> be used to encode any identifying character string uniquely associated with a resource, but reserves <location><url> for encoding the URL to the resource in its repository context. Persistent URIs that serve both as a unique identifier and a URL to the resource in its repository context should be encoded using both elements.

These guidelines require that all <identifier> URLs follow the URI specifications laid out in the DLF/NSDL Best Practices for Shareable Metadata Content.

All occurrences of the <identifier> element must contain the type attribute to indicate the type of the identifier. If a local identifier is used, it should be indicated as such using the "local" value.

See the <identifier> entry in the DLF/Aquifer Guidelines Summary of Requirements table for further information on requirements of this element, its attributes, and subelements.


Aggregator Information

<identifier> is displayed in some aggregators but is generally not used for retrieval. It may in the future be used to de-duplicate aggregations.


Relationship to DLF/NSDL Best Practices for Shareable Metadata

Identifier is addressed in the "Recommendations for classes of data elements" under "Identifiers." The DLF Aquifer guidelines recommend the use of the type attribute in lieu of prefixing identifiers, as recommended in NSDL/DLF Best Practices for Shareable Metadata.


Element Description


      Identifies the type of identifier that is recorded.

      There is no controlled list of identifier types. Suggested values include, but are not limited to the following:
      • ARK (Archival Resource Key)
      • doi (Digital Object Identifier)
      • hdl (Handle)
      • isbn (International Standard Book Number)
      • ismn (International Standard Music Number)
      • isrc (International Standard Recording Code)
      • issn (International Standard Serials Number)
      • issue number
      • istc (International Standard Text Code)
      • lccn (Library of Congress Control Number)
      • local
      • matrix number
      • music plate
      • music publisher
      • sici (Serial Item and Contribution Identifier)
      • stock number
      • upc (Universal Product Code)
      • uri (Uniform Resource Identifier)
      • videorecording identifier
      Used when additional text associated with the identifier is necessary for display.

      Include the preferred text and capitalization, but do not include delimiters such as colons.
      Signifies a canceled or invalid identifier.

      The invalid attribute should be used with the following value:
      • yes - This value is used only when recording a canceled or invalid identifier. If invalid="yes" is not present, the identifier is assumed to be valid. Do not use invalid="no."


There are no subelements for <identifier>.

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    <identifier type="music publisher">N.M. 275 Nova Music</identifier>

    <identifier type="lccn">##2001336783</identifier>

    <identifier type="doi">doi:10.1006/jmbi.1995.0238</identifier>

    <identifier type="hdl">hdl:loc.pnp/cph.3c30104</identifier>

    <identifier type="uri"></identifier>

    <identifier type="uri" displayLabel="Electronic resource (JPEG)"></identifier>

    <identifier type="isbn" invalid="yes">0877780116</identifier>

    <identifier type="uri" invalid="yes"></identifier>

    <identifier type="uri" displayLabel="Active site"></identifier>

    <identifier type="uri" displayLabel="Essays from annual reports"></identifier>

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MARC Mapping

<identifier> ≈ MARC 21 fields 010, 020, 022, 024, 856

<identifier> with displayLabel attribute ≈ MARC 21 field 856, subfields $3 or $y when using with type="uri"

See also MARC Mapping to MODS for the <identifier> element.


Dublin Core Mapping

The MODS <identifier> element, along with the <location><URL> subelement, are both mapped to <dc:identifier> in the MODS to Dublin Core Metadata Element Set Mapping Version 3.0 (June 7, 2005).

MODS example expressed in Dublin Core:



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