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--- a/ttml2/spec/ttml2.xml	Mon May 11 15:40:35 2015 +0100
+++ b/ttml2/spec/ttml2.xml	Fri May 29 11:49:48 2015 +0100
@@ -175,27 +175,32 @@
 <div2 id="model">
 <head>System Model</head>
 <p>Use of TTML is intended to function in a wider context of Timed Text
-Authoring and Distribution mechanisms that are based upon the system
+Authoring, Transcoding, Distribution and Presentation mechanisms that are based upon the system
 model depicted in <specref ref="model-graphic"/>, wherein the Timed Text Markup Language
 serves as a bidirectional interchange format among a heterogeneous collection of
 authoring systems, and as a unidirectional interchange format to a
 heterogeneous collection of distribution formats after undergoing transcoding
 or compilation to the target distribution formats as required, and where one
-particular distribution format is TTML (labeled as "DFXP" in the figure).</p>
+particular distribution format is a TTML Content Document.</p>
+<p>Two classes of processor are described. Authoring systems and validation processors
+are examples of Transformation Processors; transcoding systems and rendering processors
+are examples of Presentation Processors. A TTML Profile Document can be associated with
+a TTML Content Document or a processor, to allow each to express those features that are
+available, prohibited or required. Collectively this allows the constraints of the
+chain from authoring to presentation to be expressed in a formal language.</p>
+<p>Processors can implement the defined mapping to TTML Intermediate Documents.
+The system model depicts one such rendering processor that further maps those documents
+into HTML and CSS fragments that could be inserted into an HTML5 document for 
+display by a user agent.</p>
 <table id="model-graphic" role="example-images">
 <caption>Figure 1 &ndash; System Model</caption>
 <tbody>
 <tr>
-<td><graphic id="graphic-model" source="images/model.png" alt="System Model"/></td>
-</tr>
-</tbody>
-</table>
-<p></p>
-<ednote>
-<name>Revise System Model</name>
-<date>2014-11-27</date>
-<edtext>Update system model diagram to reflect current and expected usage. Eliminate reference to DFXP.</edtext>
-</ednote>
+<td><graphic id="graphic-model" source="images/model.svg" alt="System Model"/></td>
+</tr>
+</tbody>
+</table>
+<p></p>
 </div2> <!-- model -->
 <div2 id="example">
 <head>Document Example</head>