The NLM metadata schema is based on the metadata terms maintained by the Dublin Core Metadata Initiative, including elements, element refinements, encoding schemes, and vocabulary terms (cf. http://dublincore.org/documents/dcmi-terms/). It also incorporates some of the best practice recommendations of the DC-Library Application Profile (DC-Lib) (cf. http://dublincore.org/documents/2002/04/16/library-application-profile/). It is designed for use with electronic resources published by the Library and incorporates additional elements and qualifiers identified as requirements by NLM, most notably the addition of Permanence ratings for each resource. ¹