If you are using a DTD, you can declare an unparsed entity in it. This unparsed entity (for example, an image) will have a URI that locates the unparsed entity. The input string of the function must match the name of the unparsed entity that has been declared in the DTD. The function then returns the URI of the unparsed entity, which can then be directed to a node in the output document, for example, to an href node.
The name of the unparsed entity whose URI should be retrieved.