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xlink:href Href (Linking Mechanism)

An address or identifier that permits an XLink application to find a remote resource, for example, a URI (such as a URL or filename) that links to an online site associated with the author or the online definition of a term. As specified in XML Linking Language (XLink) Version 1.0 the value of an @xlink:href must be a valid URI.

Usage/Remarks

As specified in XML Linking Language (XLink) Version 1.0, the value of an @xlink:href must be a valid URI.
Of the many linking attributes that may be used, @xlink:href is the most important, since it provides the link. The attribute is required for those elements which must link, and it is optional on other elements only in the sense that an element need not link. If an element links, this attribute is required.
REQUIRED on many elements; click for list and usage

<graphic>, <inline-graphic>, <inline-media>, <media>

Value Meaning Behavior
Text, numbers, or special characters A pointer (which may be a URL) to a resource. Can be used to make this element a live link to a related site.
Restriction @xlink:href is required; it must be provided if the element is used.
OPTIONAL on many elements; click for list and usage

<abbrev>, <award-desc>, <award-group>, <award-id>, <award-name>, <bio>, <chem-struct>, <collab>, <collab-name>, <collab-wrap>, <conference>, <content-version>, <contrib>, <custom-meta>, <element-citation>, <email>, <ext-link>, <extended-by>, <funding-source>, <funding-statement>, <inline-supplementary-material>, <institution>, <issue-id>, <license>, <long-desc>, <mixed-citation>, <name-address-wrap>, <named-content>, <product>, <pub-id>, <related-article>, <related-object>, <resource-name>, <restricted-by>, <self-uri>, <supplementary-material>, <support-source>, <uri>, <volume-id>

Value Meaning Behavior
Text, numbers, or special characters A pointer (which may be a URL) to a resource. Can be used to make this element a live link to a related site.
Restriction @xlink:href is an optional attribute; there is no default.
Tagged Samples
Navigation for an external link (element <ext-link>) in text
The content of this <ext-link> also displays the linking URI, but the content could have displayed text such as a “Breastfeeding Manual”. 
...
<ext-link xlink:href="http://www.ncemch.org/pubs/PDFs/breastfeedingTIB.pdf" 
  ext-link-type="url">http://www.ncemch.org/pubs/PDFs/breastfeedingTIB.pdf</ext-link>
...
Link to a spreadsheet 
...
<supplementary-material id="S1" xmlns:xlink="http://www.w3.org/1999/xlink"
  xlink:title="local_file" xlink:href="pbio-0020328-t002.xls"
  mimetype="application/vnd.ms-excel">
 <label>Supporting Material</label>
 <caption>
   <p>Supplementary Microsoft Excel file (raw study data) supplied by authors.</p>
</caption>
</supplementary-material>
...
Link to a related article 
... 
<book-meta>...
<related-article related-article-type="companion" vol="2" page="e359"
  xlink:title="synopsis" xlink:href="10.1371/journal.pbio.0020359">
 <article-title>How to Make a Mother in Five Easy Steps</article-title>
</related-article>
</book-meta>...