Element Domain Specialization Step 4: Integrate The Module Into a Document Type Shell
Step 4-2. Create a Test Topic
Step 4-4. Update Type-Specific Parameter Entities in Shell DTD

Step 4-3. Add .ent File to Shell

Add the domain entity declaration (.ent) file to concept.dtd. In your copy of concept.dtd, find the comment that says "DOMAIN ENTITY DECLARATIONS" and following this comment add the following parameter entity declaration and reference: 
<!ENTITY % xml-d-dec     
   SYSTEM "xmlDomain.ent"                                                
>
%xml-d-dec;

The system identifier needs to reflect the actual location of the xmlDomain.ent file. The example assumes that concept.dtd and xmlDomain.ent are in the same directory.