2.3 Content Model Declarations Content model declarations are made within the declaration for the element to which they apply. The simplest content model is empty, which indicates that the parent element has no sub-elements and no character data content.
Empty element indicates that an element is empty.
This would not allow the Species element to contain any text or sub-elements. Using the Any content model is much like using the Empty content model. This would allow the Species element to contain text and any sub-elements an author desired. The Reference content model allows an element to specify other elements which it may contain, as well as their quantity. To define content models that permit or require the use of more elements, the Any, Mixed, Choice, or Sequence content models should be used as appropriate.