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It is desirable to express some semantics of the Link href in another attribute "rel". In the current first version of UBIF, the list of values for Link rel should be rather restrictive to encourage adoption and simplify the understanding of the standard. It is, however, explictly anticipated that the list should be expanded in the future, possibly even made unconstrained (similar to it being unconstrained in xhtml).

[[http://www.w3.org/TR/2004/WD-xhtml-modularization-20040218][Modularization of XHTML 1.0 - 2nd Ed. Working Draft 18 February 2004]] under "4.3. Attribute Types" lists recognized values for !LinkTypes that may be used in the rel attributes. (Notes: The values in xhtml are defined to be case-insensitive. Closely related to "rel" is the "rev" attribute, which substitutes rel in the case of inverse relationships.)

Most of the recommended values for !LinkTypes seem to be inapplicable to UBIF data exchange, or further study is needed where they may apply (Stylesheet, Contents, Index, Glossary, Copyright, Chapter, Section, Subsection, Appendix, Bookmark). Some types (Start, Next, Prev) are applicable where the provider defines a single normative sequence of objects, but no example for an application requiring this in biodiversity objects is known. The following two values for the xhtml Link rel attribute are probably closest to the purposes for which

	* _Alternate:_ Designates substitute versions for the document in which the link occurs. When used together with the hreflang attribute, it implies a translated version of the document. When used together with the media attribute, it implies a version designed for a different medium (or media). 
	* _Help:_ Refers to a document offering help (more information, links to other sources information, etc.) 

In xhtml 2.0 an additiona value of _Meta_ for document meta information (expressed human readable or perhaps as RDF) is proposed.

*Examples (xhtml)*
<verbatim>
  <link rel="alternate" type="text/html" title="HTML version" href=",html" />
  <link rel="alternate" type="application/xhtml+xml" title="XHTML version" href=",xhtml" />
  <link rel="help" href="../Help/siteindex" title="Site Index" />
</verbatim>

[[http://www.w3.org/TR/xlink/][XLink]] does not use a "rel" attribute. The xlink:role expresses (in most cases or always?) the format type of the linked resource (pdf, schema for xml documents, etc., all expressed as a uri). However, xlink:arcrole expresses the role of the link (= arc) between two resources and is closer to xhtml:rel. No vocabulary is defined for role or arcrole in XLink itself, only that they must be a uri. User groups using XLink can thus define their own vocabulary. 

Based on the information above, and the argument made in ObjectTypePattern about a separation of instance identifiers (specific representation) and abstract identifier (refers to a physical object or abstract concept that may be represented in multiple ways, e.g. a specimen expressed in "DarwinCore or ABCD unit), the missing role or rel value seems to be 

	* _About:_ Refers to a resource that is non-digital or abstract and for which the current object instance is one of possibly multiple representations. 

The three values "Alternate", "Meta", and "About" are proposed for UBIF 1.0. An explicit "Help" (rather than subsuming this under Meta), and perhaps a seeAlso borrowed from RDF are under considerations, opinions/discussions are welcome!

-- Main.GregorHagedorn - 14 Mar 2005
---

New proposed additions:

Latest for lastest version, Previous for previous version of an object.

-- Main.GregorHagedorn - 19 Mar 2005

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[[http://www.w3.org/TR/xlink/][XLink]]  does not use a "rel" attribute, the "role" attribute is a close equivalent.  No vocabulary is defined in XLink itself, only the fact that role must be a uri is specified. User groups using XLink can thus define their own vocabulary.
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The three values "Alternate", "Help", and "About" are proposed for UBIF 1.0
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