These keywords denote the marked sections content model, and allow you to change it from the default.

When an XML parser is processing a document, it keeps track of the “content model”.

The content model describes the content the parser is expecting to see and what it will do with that content.

The CDATA content model is one of the most useful.

CDATA is for “Character Data”. When the parser is in this content model, it expects to see only characters. In this model the < and & symbols lose their special status, and will be treated as ordinary characters.

<para>Here is an example of how to include some text that contains
  many <literal>&lt;</literal> and <literal>&amp;</literal>
  symbols.  The sample text is a fragment of
  <acronym>XHTML</acronym>.  The surrounding text (<para> and
  <programlisting>) are from DocBook.</para>

<programlisting><![CDATA[<p>This is a sample that shows some of the
  elements within <acronym>XHTML</acronym>.  Since the angle
  brackets are used so many times, it is simpler to say the whole
  example is a CDATA marked section than to use the entity names for
  the left and right angle brackets throughout.</p>

    <ul>
      <li>This is a listitem</li>
      <li>This is a second listitem</li>
      <li>This is a third listitem</li>
    </ul>

    <p>This is the end of the example.</p>]]></programlisting>

When the keyword is INCLUDE, then the contents of the marked section will be processed. When the keyword is IGNORE, the marked section is ignored and will not be processed. It will not appear in the output.

<![INCLUDE[
  This text will be processed and included.
]]>

<![IGNORE[
  This text will not be processed or included.
]]>

By itself, this is not too useful. Text to be removed from the document could be cut out, or wrapped in comments.

It becomes more useful when controlled by parameter entities, yet this usage is limited to entity files.

For example, suppose that documentation was produced in a hard-copy version and an electronic version. Some extra text is desired in the electronic version content that was not to appear in the hard-copy.

Create an entity file that defines general entities to include each chapter and guard these definitions with a parameter entity that can be set to either INCLUDE or IGNORE to control whether the entity is defined. After these conditional general entity definitions, place one more definition for each general entity to set them to an empty value. This technique makes use of the fact that entity definitions cannot be overridden but the first definition always takes effect. So the inclusion of the chapter is controlled with the corresponding parameter entity. Set to INCLUDE, the first general entity definition will be read and the second one will be ignored. Set to IGNORE, the first definition will be ignored and the second one will take effect.

<!ENTITY % electronic.copy "INCLUDE">

<![%electronic.copy;[
<!ENTITY chap.preface	SYSTEM "preface.xml">
]]>

<!ENTITY chap.preface "">

<!ENTITY % electronic.copy "IGNORE">