When set on a section, introductory indicates that it is meant as introductory material that need not be linked to from the table of contents. The abstract, SotD, and ToC sections automatically fall into this category (but you need not worry about flagging them as such).

If you do wish all the introductory sections to be present in the ToC, see tocIntroductory.

<section class="introductory">
  <p>This document can be folded into a paper airplane.</p>
</section>