Specify the types of allowable values for given elements, categories or fields.

• Dublin Core
• Author-generated Dublin Core Metadata for Web Resources: A Baseline Study in an Organization - Jane Greenberg, et al, Journal of Digital Information, November 2001
• Dublin Core Metadata Element Set
• Keeping Dublin Core Simple: Cross-Domain Discovery or Resource Description? - Carl Lagoze, D-Lib Magazine, January 2001
• Metadata for the masses - Paul Miller describes Dublin Core and means by which it can be implemented
• Namespaces in XML - (W3C Recommendation) XML namespaces provide a simple method for qualifying element and attribute names used in Extensible Markup Language documents by associating them with namespaces identified by URI references
• Namespaces in XML 1.1 - (W3C Working Draft as of 2022-09-11) incorporates errata corrections and provides a mechanism to undeclare prefixes
• Ruby Annotation - (W3C Working Draft as of 2001-05-08) defines an XHTML module for markup of ruby, which are short runs of text alongside the base text, typically used in East Asian documents to indicate pronunciation or to provide a short annotation
• SemanticWeb.org - Semantic Web Community Portal
• Topic Maps (ISO 13250) - application of HyTime that allows semantics to be associated with part or all of a networked resource
• XML Schema: Formal Description - (W3C Working Draft as of 2001-10-22) declarative system for describing and naming XML Schema information, specifying XML instance type information, and validating instances against schemas
• XML Schema Part 0: Primer - (W3C Recommendation) non-normative document intended to provide an easily readable description of the XML Schema facilities and is oriented towards quickly understanding how to create schemas using the XML Schema language
• XML Schema Part 1: Structures - (W3C Recommendation) specifies the XML Schema definition language, which offers facilities for describing the structure and constraining the contents of XML 1.0 documents
• XML Schema Part 2: Datatypes - (W3C Recommendation) proposes facilities for defining datatypes to be used in XML Schemas and other XML specifications