B2B Integration : A Practical Guide to Collaborative E-commerce

Document type definitions (DTD)
Extensible Markup Language (XML) 171
An XML document is essentially a structured medium for storing
information. But how does an application identify which XML document
is valid or not? In order to assess the validity of an XML document,
the application needs to establish exactly to which structure the
information within the document must adhere. This is accomplished
with a schema, which is a model used to describe the structure of
information within an XML document. Schemas are used in XML to
model a class of data.
One of the principal uses of structured information is to enable data
interchange. Different industries create consortia to specify the content
model on which they all agree and which they use to mark up their
information so that they can share it with each other easily and efficiently.
In the context of structured information, that content model is defined
as a DTD or a schema.
DTDs are a means of establishing a schema for XML documents.
The purpose of a DTD is to define the legal building blocks of an
XML docment. It defines the document structure with a list of legal
elements. A DTD can be declared inline in an XML document, or as an
external reference.
Example of a DTD:
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