The ExternalParameterEntityDecl element represents a parameter entity definition where the entity definition is contained externally to the current message. It is a named element and has a child of type SystemId. It can also have a child of type PublicId. The name of the entity does not include the percent sign %. In XML an external parameter entity declaration takes the form:
<!ENTITY % bookDef SYSTEM "BOOKDEF.DTD">
This represents an ExternalParameterEntityDecl element of name bookDef with a single child of type SystemId with a string value of BOOKDEF.DTD.