As I recommended in DITA: Specializations (task, concept, reference)?, author in the base topic type. You won't find yourself doing acrobatics to combine topics. However, you may want to combine topics in order to create nested subheadings.

Combining topics with the chunk attribute

The chunk="to-content" attribute allows you to merge child topics into a parent topic in your navigation. You add the chunk attribute on a parent topicref.

  <topicref href="parent.dita" chunk="to-content">
  <topicref href="child1.dita" toc="no"/>
  <topicref href="child2.dita" toc="no"/>
  </topicref>
   

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If you want the child topics to actually appear in the table of contents, use the "yes" value.

Although the chunk attribute works easily enough in the TOC area, it's problematic with linking. Chunking creates issues with relationship tables. (See DITA: Limitations with the chunk=”to-content” attribute in relationship tables.)

Combining topics by nesting elements

A better way to combine topics is by nesting elements. When you nest elements, you add multiple taskbody or conbody elements inside each other. Here's an example of how nest task elements.

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE task PUBLIC "-//OASIS//DTD DITA Task//EN" "task.dtd">

<task id="task_0">
    <title>Fixing a flat tire</title>
    <shortdesc></shortdesc>
    <taskbody>
        <context>
            <p>This is an overview about the task of fixing a flat tire...</p>
        </context>

    </taskbody>

    <task id="task_1">
        <title>Remove the tire</title>
        <shortdesc/>
        <taskbody>
            <context>
                <p>Context for the current task</p>
            </context>
            <steps>
                <step>
                    <cmd>Task step.</cmd>
                </step>
            </steps>
        </taskbody>
    </task>

        <task id="task_2">
            <title>Inspect the tire</title>
            <shortdesc/>
            <taskbody>
                <context>
                    <p>Context for the current task</p>
                </context>
                <steps>
                    <step>
                        <cmd>Task step.</cmd>
                    </step>
                </steps>
            </taskbody>
        </task>

            <task id="task_3">
                <title>Insert a new tube</title>
                <shortdesc/>
                <taskbody>
                    <context>
                        <p>Context for the current task</p>
                    </context>
                    <steps>
                        <step>
                            <cmd>Task step.</cmd>
                        </step>
                    </steps>
                </taskbody>

            </task>

                <task id="task_4">
                    <title>Reinstall the tire</title>
                    <shortdesc/>
                    <taskbody>
                        <context>
                            <p>Context for the current task</p>
                        </context>
                        <steps>
                            <step>
                                <cmd>Task step.</cmd>
                            </step>
                        </steps>
                    </taskbody>
                </task>

</task>

With nesting concepts, here's what that looks like

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">

<concept id="concept_1">
 <title>License overview</title>
 <conbody>
  <p>Here's some overall information about your license....</p>
 </conbody>

 <concept id="concept_2">
  <title>Getting your license</title>
  <conbody>
   <p>Inforamtion about activating your license...</p>
  </conbody>

 <concept id="concept_3">
  <title>Registering your license</title>
  <conbody>
   <p>Information about registering your license...</p>
  </conbody>
</concept>

</concept>

Basically, just bookend the individual task or concept elements inside one task or concept element that functions as a container.

Using an outputclass for a third-level section

You can also "cheat" to combine topics against the design of the DITA structure by crafting an outputclass attribute to look like a third-level heading: like this:

         <p outputclass="level3">Sample heading</p>
     

Then add p.level3 class to the webhelp stylesheet. Assuming your OxygenXML install directory begins in your /Applications folder, the path would be as follows:

/Applications/oxygenAuthor/frameworks/dita/DITA-OT/plugins/com.oxygenxml.webhelp/oxygen-webhelp/resources

Open the webhelp_topic.css file, scroll down to the bottom, and add something like this for your style:

.level3 {
  color: #444;
  font-size: 16px;
  font-style: italic;
}
     

This is more of a workaround hack than any good technique. Sometimes you just need a third-level heading, though, and this works.

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