title: What is the _root_ field in schema.xml?
body: <p>I am reading solr example schema.xml. And I found a field named <code>_root_</code>. I have never seen it before. I don't know what it does. </p>
  
  <pre><code>&lt;!-- points to the root document of a block of nested documents. Required for nested document support, may be removed otherwise --&gt;
  &lt;field name="_root_" type="string" indexed="true" stored="false"/&gt;
  </code></pre>
  
  <p>Is it a new feature of solr? What are the <code>nested documents</code>? And for what situation I should use this field?</p>
  
  <p>My solr version is 4.6.</p>
  
  <p>Thanks in advance.</p>