The DTD (Document Type Definition) contains the rules that govern how the XML data is constructed.
It determines the names of the elements in the data, where they may appear and how frequently, and also defines the underlying "metadata", such as attributes on elements.
Although the DTD is written in a text editor, it is most readable when displayed in a "tree" structure through which you can navigate to determine which elements are allowed where. "Near & Far" software is able to produce this tree.
Click on one of the links below to view the corresponding dictionary.