The definition of an XML document, which identifies the XML tags and includes the rules for their interrelationships; for example, there must be at least one B tag within every tag A, or there can only be B tags within an A tag, and so forth. This is like any common relationship in a database, such as each customer can be associated to order records, but not to payroll records, or each department must have one or more employees.
Residing within the document itself, an XML schema is commonly used to verify the integrity of the content. Various recommendations for an XML schema were submitted to the W3C, and a standard was approved in May 2001 that included the ability to define data by type (date, integer, etc.) just like database records. See
XML,
W3C XML Schema,
RELAX NG,
Schematron,
SOX,
XDR,
DCD,
XML-Data,
DDML and
XML vocabulary.