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xlink:role Role of the Link

A URI reference pointing to some resource that describes the role or function of a link.
OPTIONAL on many elements; click for list and usage

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

Value Meaning
Text, numbers, or special characters The URI of a resource that describes the role, reason, or rationale for the link.
Restriction @xlink:role is an optional attribute; there is no default.
Tagged Sample

URI reference explaining the purpose of an external link (in this instance, to XML coding) 

...
<ext-link ext-link-type="url" xlink:type="simple"
  xlink:role="http://www.mulberrytech.com/walkthewalk.html"
  xlink:title="XML-based slideshow application"
  xlink:actuate="onRequest" xlink:show="new"
  xlink:href="http://www.mulberrytech.com/slideshow/index.html">
 Mulberry Slideshow XML</ext-link>
...