The $outline variable contains the table of content of a document.
Using the $outline variable generates HTML code.
For example:
<ol>
<li class="outline-entry-1">Fluid Topics Installation
<ol>
<li class="outline-entry-2">Prerequisites
<ol>
<li class="outline-entry-3">Hardware Prerequisites</li>
<li class="outline-entry-3">Software Prerequisites</li>
<li class="outline-entry-3">Installation Types</li>
</ol>
</li>
<li class="outline-entry-2">Architecture Examples</li>
</ol>
</li>
</ol>
Each tree level is annotated with a CSS class indicating the topic depth in this structure (.outline-entry-{depth}).