Unparsed entities are a way to add pointers to your document that point at different kinds of data; the most common application would be for nontextual objects like graphics, video, and other multimedia. To use an unparsed entity, you have to do four things:
Here's an example laid out step-by-step:
<!-- 1 -->
<!NOTATION vrml PUBLIC "VRML 2">
<!-- 2 -->
<!ENTITY Antarctica SYSTEM "http://www.antarctica.net" NDATA vrml>
<!-- 3 -->
<!ATTLIST World src ENTITY #REQUIRED>
<!-- 4 -->
<World src='Antarctica'>