Extensibility and Domains of Applicability

Key to the nature of XML is its extensibility. XML elements can be defined as they are needed to fulfill the needs of specific document uses. Network management is one such area of use or domain, and subdomains might be defined for the management of a type of network element (e.g., a router) or even for a specific make and model of a network element.

It is important to note that the definition of new XML elements is not the same as the definition of new tags or syntaxes within XML. Tags and syntaxes are standardized, meaning that all XML documents can be successfully parsed by any correctly implemented XML engine regardless of the domain to which the document applies. The semantics of an XML element may, however, only be understood within its domain of applicability.

The documents used within a specific domain will use a well-known set of XML elements, tags, and markup data. Knowledge of this information is useful to implementers since it governs the amount of code they have to write to construct and parse XML and to interpret XML elements. XML documents for a domain are described in a document type definition (DTD) and, conversely, a document identifies the domain to which it belongs by indicating the DTD. Note that DTDs may be nested as subsets of other DTDs so that a document that conforms to a child DTD will also conform to the parent.