Notation declarations provide labels for content in a document. The XML Recommendation [XML] requires that the DTD contain all notation declarations for a document. You use notations to identify file types or for a variety of other tasks. Notations can identify the following items by name:
Notation declarations provide a name for the notation, which you can then use in entity and attribute-list declarations and specifications. Notation declarations can also provide a name for an external identifier for the notation that allows an XML processor or its client application to locate a helper application capable of processing data in the specified notation.
Only one notation declaration can declare a particular name. The XML Recommendation stipulates that XML processors must provide applications with the name and external identifier or identifiers of any notation declared and referred to in an attribute value, attribute definition, or entity declaration. XML documents, however, can declare and refer to notations for which notation-specific applications are not available on the system where the XML processor or application is running.