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Introduction to XML For Web Developers
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The REQUIRED flag specifies that though there is no default value provided by the DTD,
the attribute when actually implemented in an XML document must define a value. for
example, suppose you wanted to define a standard PAGE_AUTHOR element that
could be added to every page on any site that used it. Your intent is to
make sure that every author provides contact information for bugs and broken links.
However, you won't know in advance what the default values should be because everyone
who implements your DTD will have different personal information. Thus, you can make the
contact information attributes required, while not providing defaults.
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When you use the IMPLIED default, you will provide a default value for the document author.
If the document author does not override your default, your default will be used.
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Sometimes you will want to provide a default value that the document author may not modify.
In that case, you will use FIXED.
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