Schema 3 allows a much looser content model than previous versions. For example:

<molecule xmlns='http://www.xml-cml.org/schema'>
  <atom>
    <bond>
      <molecule />
    </bond>
  </atom>
</molecule>

is schema-valid although probably meaningless - what kind of atom is it, what is the bond between?

We use conventions to put restrictions on the content model. The links below contain information about how to put together documents which conform to a particular conventions and examples of conformant documents.