1. The <concept> element

Create a <concept> Opens in new window element when you need to provide your reader with background information which must be absorbed in order to understand the rest of the document.
Example:
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<concept id="what_is_a_cache">
  <title>What is a cache?</title>

  <shortdesc>Everything you'll ever need to know about
  <term>cache</term>s.</shortdesc>

  <conbody>
    <p>In computer science, a cache is a temporary storage area where
    frequently accessed data can be stored for rapid access.</p>
  </conbody>

  <related-links>
    <link format="html" href="http://en.wikipedia.org/wiki/Cache"
          scope="external">
      <linktext>Wikipedia definition of a cache</linktext>
    </link>
  </related-links>
</concept>
Notice how the structure of a <concept> element is similar to the structure of a <topic> element. Moreover, a <conbody> Opens in new window element has almost the same content model as a <body> Opens in new window element.
The above example is rendered as follows: