In some cases it might be handy to provide multiple documents within one XML import file. To do so, you have to encapsulate the <document> elements in the parent element <documents>:
<?xml version="1.0" encoding="UTF-8"?>
<documents xmlns="http://www.sophoracms.com/import/4.2"
           xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <document nodeType="sophora-content:story">
    <properties>
      [...]
    </properties>
    <childNodes>
      [...]
    </childNodes>
    <resourceList>
      [...]
    </resourceList>
    <fields>
      [...]
    </fields>
    <instructions>
      [...]
    </instructions>
  </document>
  [...]
  <document nodeType="sophora-content:video">
    [...]
  </document>
  [...]
  <document nodeType="sophora-content:image">
    <properties>
      [...]
    </properties>
    <childNodes>
      [...]
    </childNodes>
    <resourceList>
      [...]
    </resourceList>
    <fields>
      [...]
    </fields>
    <instructions>
      [...]
    </instructions>
  </document>
</documents>