The major difference

If there is a namespace declaration anywhere in the XML file, it is available for that element and all elements in its child tree.

Unqualified: If you declare a namespace without using a prefix (unqualified), the default namespace is changed. This means, that all elements without a namespace prefix will be in that specific namespace in the child tree of that element.

Qualified: When using qualified namespace declaration, you define a namespace prefix (like a reference) and only those elements will be in that namespace, that have the specified prefix. The default namespace is not changed, therefore elements without prefixes remain in empty namespace.

Alexander Randall