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DocBook comes in a couple of different flavors--including both XML and SGML formats. This means that you may use either the SGML grammar/rules when adding markup, or you may use the XML grammar/rules. Either way you may only use one set of rules throughout your document, and you must define which one you are using at the top of your document.
The LDP prefers the XML flavor of DocBook. There are a number of reasons for this including the following:
Libraries for handling XML files are developing at a rapid pace. These utilities may make it easier for new authoring tools to become available.
Many popular word processing programs are now creating XML output. While it may not be DocBook XML, this does make it easier for application writers to either add DocBook XML support, or provide some method of translating between their format and DocBook XML.
Everyone else is doing it. While this might not be a real reason, it allows the LDP to keep up-to-date with similar projects. Tools, procedures, and issues can be worked out in a common framework.
Still not convinced? Fortunately the LDP does accept a number of other file formats for input. The list of accepted markup languages can be found in Section 5.4