"extension" is used to provide for additional information not covered by MODS. It may be used for elements that are local to the creator of the data, similar to MARC 21 9XX fields. In addition, it may be used to extend MODS for various purposes when another XML schema may handle the type of information. For instance, holdings information for the resource may be included in "extension" with a link to an XML holdings schema (such as the Z39.50 XML Holdings Schema). There are no subelements defined for "extension".