XML - HTML and XHTML: The Definitive Guide - page 856

IDREF or IDREFS
Indicate that the attribute accepts an ID defined elsewhere in the
document via an attribute of type ID . You use the ID type when de-
fining IDs; you use IDREF and IDREFS when referencing a single ID
and a list of IDs, respectively.
ENTITY or ENTITIES
Indicate that the attribute accepts the name or list of names of un-
parsed general entities defined elsewhere in the DTD. The defin-
ition and use of unparsed general entities is covered in section
15.3.2 .
NMTOKEN or NMTOKENS
Indicate that the attribute accepts a valid XML name or list of
names. These names are given to the processing application as
the value of the attribute. The application determines how they are
used.
In addition to these keyword-based types, you can create an enumer-
ated type by listing the specific values allowed with this attribute. To
create an enumerated type, list the allowed values, separated by pipe
characters and enclosed in parentheses, as the type of the attribute. For
example, here is how the method attribute for the <form> tag is defined in
the HTML DTD:
method (get|post) "get"
The method attribute accepts one of two values, either get or post ; get is
the default value if nothing is specified in the document tag.