文件型態定義(Document Type Definition) 

Document  Type Declarations

A document type declaration is placed in the xml document's prolog and begin with
<! DOCTYPE and ends with >
The Document  type declaration can use two method

1.external subset
2.internal subset

the internal subset might look like that

<!DOCTYPE  Message [
       <!ELEMENT Message (#PCDATA)>


the external subset might look like that

<!DOCTYPE  Message SYSTEM "myDTD.dtd">

Document  Type Declarations

DTD with element type declarations (ELEMENTs)
  <!ELEMENT my Element (#PCDATA)>

keyword PCDATA specifies that the element must contain parsable character data