All XML documents must have at least one root element to be well formed.
The root element, also often called the document tag, must follow the
prolog (XML declaration plus DTD) and must be a nonempty tag that
encompasses the entire document.
Generally, you are supposed to match the root element name to the
DTD declaration. For example, this declaration
<!DOCTYPE Instrument PUBLIC
"-//NASA//Instrument Markup Language 0.2//EN"
implies that "Instrument" is my root element. (This rule isn't enforced, but
it is a convention.)