edx-platform/cms/templates/ux/reference/outline_add-subsection.html

<div class="add-subsection add-item">
  <a href="#" class="button button-new" data-category="" data-parent="" data-default-name="New Subsection">
    <i class="icon icon-plus"></i> New Subsection
  </a>
</div>
<!-- /add-subsection -->