add note to describe DAPT implementation types (#245)

* add note to describe DAPT implementation types

* address review comments

index.html

@@ -2548,6 +2548,22 @@ <h3>Conformance of DAPT Processors</h3>
         <a href="https://www.w3.org/TR/ttml2/#profile">profile</a> vocabulary of [[TTML2]] in the
         <dfn data-cite="TTML2#terms-processor-profile">processor profile</dfn> defined
         in <a href="#profiles"></a>.</p>
+        <aside class="note">The provisions in this specification are expressed in terms of [[TTML2]] <a>content processor</a> types,
+        using terms like <a>content processor</a>, <a>presentation processor</a>, <a>validation processor</a>,
+        or <a>transformation processor</a>. They may be understood with the mapping below:
+        <ul>
+          <li>A DAPT tool that would display the content of a DAPT document, in any form:
+            for example, just rendering the text, event by event; or rendering the text of all the events
+            all at once along a timeline; or one doing the audio mixing, would be considered
+            a <a>presentation processor</a>. A <a>presentation processor</a> can also support additional rendering features
+            from TTML2, such as styles for DAPT documents that use that.</li>
+          <li>A DAPT authoring tool, producing DAPT documents from scratch or from an other document,
+            would be considered a <a>transformation processor</a>. It may also be considered as
+            a <a>presentation processor</a> if it does any form of rendering of the text or the audio.</li>
+          <li>A DAPT tool that would indicate if a DAPT document is compliant to this specification
+            would correspond to a <a>validation processor</a>.</li>
+        </ul>
+        </aside>
     </section> <!-- processor-profile-conformance-section -->
   </section> <!-- conformance -->