XML in a Nutshell, 3rd Edition

Notations

The notation syntax of XML provides a way for the document author to specify an external unparsed entity’s type within the XML document’s framework. If an application requires access to external data that cannot be represented in XML, consider declaring a notation name and using it where appropriate when declaring external unparsed entities. For example, if an XML application were an annotated Java source-code format, the compiled bytecode could then be referenced as an external unparsed entity.

Notations effectively provide metadata, identifiers that applications may apply to information. Using notations requires making declarations in the DTD, as described in Chapter 3. One use of notations is with NOTATION-type attributes. For example, if a document contained various scripts designed for different environments, it might declare some notations and then use an attribute on a containing element to identify what kind of script it contained:

<!NOTATION DOS PUBLIC "-//MS/DOS Batch File/">
<!NOTATION BASH PUBLIC "-//UNIX/BASH Shell Script/">
<!ELEMENT batch_code (#PCDATA)*>
<!ATTLIST batch_code 
    lang NOTATION (DOS | BASH)>
. . . 
<batch_code lang="DOS">
  echo Hello, world!
</batch_code>

Applications that read this document and recognized the public identifier could interpret the foreign element data correctly, based on its type. (Notations can also have system identifiers, and applications can use either approach.)

Categorizing processing instructions is the other use of notations. ...