flexform example: select content element

<content_element>
  <TCEforms>
    <label>Inhaltselement</label>
    <config>
      <type>group</type>
      <internal_type>db</internal_type>
      <allowed>tt_content</allowed>
      <size>1</size>
      <minitems>1</minitems>
      <maxitems>1</maxitems>
    </config>
  </TCEforms>
</content_element>