help with entites in a schema

All,
  I'm a newbie to XML and need some help.  I'm designing a schema for one of our data formats and have a question about entities.  How do I incorporate the "use" of an unparsed entity, in this case the entity is an external HDF5 file which holds the data.  Do I need to include the entity at all?
 
We use Paraview to visualize most of our data, Paraview loads the XML file and reads the data from the HDF file without issue.
 
Here is XML file:
<?xml version="1.0" encoding="UTF-8"?>

<!DOCTYPE Xdmf [ 

<!ENTITY HeavyData "C:/Documents and Settings/John Vines/My Documents/Altova Projects/XdmfSchema/cone.h5">

]>

<Xdmf xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="C:\Documents and Settings\John Vines\My Documents\Altova Projects\XdmfSchema\XdmfSchemav2.xsd"> 

<Domain>

<Grid Name="Unnamed">

<Topology Type="TRIANGLE"

Dimensions="468">

<DataStructure

DataType="Int"

Dimensions="468 3"

Format="HDF">

C:/Documents and Settings/John Vines/My Documents/Altova Projects/XdmfSchema/cone.h5:/Unnamed/Connections

</DataStructure>

</Topology>

<Geometry Type="XYZ">

<DataStructure

DataType="Float"

Precision="4"

Dimensions="236 3"

Format="HDF">

C:/Documents and Settings/John Vines/My Documents/Altova Projects/XdmfSchema/cone.h5:/Unnamed/XYZ

</DataStructure>

</Geometry>

</Grid>

</Domain>

</Xdmf>

 

Thanks in advance,

John