When creating XML documents, you must work from a design that defines the structure of the document. This strengthens the structure and design. If you need to send the document to others, especially from other companies, they need the document format. Without a formal design of the XML document format, it is extremely difficult to write modules to process the document. The XML standard supports several design standards documents:

• Document type definition (DTD) — the oldest document standard used mainly in legacy applications. SQL Server doesn’t recognize, use, or support DTD files.
• XML Data Reduced (XDR) — Alternative language option, typically used for inline schemes (schemes included in the file with the document).
• XML schema (XSD) — it is the current prevailing document design standard and SQL Server supports this design standard.

Checkout my article here for an complete overview of XSD.

This article is published on SSWUG.org.