DTD provides syntax for loading XML contained in other files - Interlinks.

This takes the form of an DTD Entity.

books.xml

<bookstore>
	<book>
	<name>All about XML</name>
		<price>$20</price>
	</book>
	
	<book>
		<name>Learning XQuery</name>
		<price>$15</price>
	</book>
</bookstore>

Link to original

This books.xml file can then be imported with:

<!ENTITY bookstore SYSTEM "books.xml">

And referenced with &books;

Example:

<!DOCTYPE stores [
	<!ENTITY bookstore SYSTEM "books.xml">
	<!ENTITY coffeeshop SYSTEM "coffee.xml">
]>
 
<shops>
	&bookstore;
	&coffeeshop;
</shops>