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Whether that content is actually usable by your application is another question — just like a syntactically correct program may be full of logic bugs.
I once saw an attempt to use XSD to validate relationships in a complex object graph; the developer gave up after creating a schema document.
Let validation do what it can, but ultimately your program must explicitly verify that an XML file contains the correct data.
The Document Type Definition is part of the XML specification.
And that brings up the second case: if you get XML from someone who doesn't know the rules.
For example, XML generated using simple string output in a Windows environment will probably be encoded in .
Normally, this isn't an issue, especially if the XML is both produced and processed within the same organization.
A “valid” document, by comparison, is one where the document's corresponds to some specification.
A document may be well-formed — and usable — but not valid.