The following expressions appear within a DTD document.  

DTD Expression	Description
CDATA	Data Type : Character Data
ID	Data Type : Unique Identifier
IDREF	Data Type : Reference to a Unique Identifier
IDREFS	Data Type : Reference to a 0-n Unique Identifiers
ENTITY	Data Type : Reference to an ENTITY Declaration
ENTITIES	Data Type : Reference to an 0-n ENTITY Declarations
NMTOKEN	Data Type : Named Token
NMTOKENS	Data Type : 0-n Named Tokens
?	Element cardinality : Optional (0-1)
*	Element cardinality : Zero to Many (0-n)
+	Element cardinality : One to Many (1-n)
,	Sequence : separates items within a sequence
|	Choice : separates items within a choice
-->	Ends a comment
<!--	Starts a comment
ANY	Declares an attribute on an Element
ATTLIST	Declares an attribute on an Element
EMPTY	Declares an attribute on an Element
DOCTYPE	Starts a DTD declaration
ELEMENT	Defines an XML Element
ENTITY	Defines an ENTITY, which can be used as a replacement token
NOTATION
#PCDATA	Allows mixed content to be defined
#REQUIRED	Attribute property : Attribute is required
#FIXED	Attribute property : Attribute must have a specific value
#IMPLIED	Attribute property : Attribute is optional (0-1)
SYSTEM	ENTITY property : The entity value is to be loaded from a uri
PUBLIC	ENTITY property : The entity value is refered to by a public identifier, or can be loaded from a uri