The reports are defined by the <reporting> section of the corporate POM. These only appear in a module's site if there is a <menu ref="reports"/> link; generally this is the case for child modules and is not the case for parent modules.

At the time of writing, the reports defined were:

<reporting>
  <plugins>
    <plugin>
      <groupId>org.apache.maven.plugins</groupId>
      <artifactId>maven-project-info-reports-plugin</artifactId>
      ...
    </plugin>

    <plugin>
       <groupId>org.codehaus.mojo</groupId>
       <artifactId>cobertura-maven-plugin</artifactId>
    </plugin>

    <plugin>
      <groupId>org.apache.maven.plugins</groupId>
      <artifactId>maven-surefire-report-plugin</artifactId>
    </plugin>

    <plugin>
      <groupId>org.apache.maven.plugins</groupId>
      <artifactId>maven-checkstyle-plugin</artifactId>
      ...
    </plugin>

    <plugin>
      <groupId>org.apache.maven.plugins</groupId>
      <artifactId>maven-pmd-plugin</artifactId>
      ...
    </plugin>

    <plugin>
       <groupId>org.codehaus.mojo</groupId>
       <artifactId>taglist-maven-plugin</artifactId>
    </plugin>

    <plugin>
      <groupId>org.apache.maven.plugins</groupId>
      <artifactId>maven-jxr-plugin</artifactId>
      ...
    </plugin>

    <plugin>
      <groupId>org.apache.maven.plugins</groupId>
      <artifactId>maven-javadoc-plugin</artifactId>
      ...
    </plugin>
  </plugins>
</reporting>

The maven-project-info-reports-plugin plugin generates the submenu under the "Project Information" menu item:

The remaining plugins each generate a submenu item under the "Project Reports" menu item: