It contains specs for each element, including what the element s attributes are, what values the attributes can take on, and what elements can be contained in others. DTDs are also used by software to validate the syntax of XML documents. XPCI is a set of DTDs that defines the terminology, transaction enveloping, and XML message definitions. This DTD defines the transaction-enveloping scheme.
Every message includes transaction information.
Each message has an associated DTD that defines the vocabulary of the message.