DOCTYPE Declaration

• Notation:
Using System identifier: The system identifier "hello.dtd" gives the address (a URI reference) of a DTD for the document. Greeting is the name of the root element of the XML document as it is defined in the DTD
<!DOCTYPE greeting SYSTEM "hello.dtd">

Using Public Identifier:
<!DOCTYPE greeting PUBLIC "Public Identifier" "hello.dtd">

Using Inline (Internal Subset) definition: Allows to contain Markup Declarations in the XML Document
<!DOCTYPE test [<!ELEMENT test (#PCDATA)> ]>

References:

Controlling the DOCTYPE and XML Declaration

Drop the <!DOCTYPE >

The Escape Characters