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Hour 23. Using XML to Describe Data > Document Type Definitions (DTDs)

Document Type Definitions (DTDs)

When XML data files are created, it is possible to include a set of rules that the data is to follow. To explain this, consider an expanded definition of an email object that includes the carbon copy (or cc) and blind carbon copy (or bcc) data elements. The general rules of an email are the following:

  • The message must be sent to one or more receivers. Therefore, there can be one or more recipients of the message.

  • The message can have only one point of origin; therefore, there can be one and only one sender.

  • There can be one and only one date stamp.

  • The cc is optional and there can be more than one cc recipient.

  • The bcc is optional and there can be more than one bcc recipient.

  • A subject is not required, but if it exists, there can be only one.

  • A body is not required, but if it exists, there can be only one.


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