Paulette I think you need to take some XML training in order to fully get your arms around this. For XML applications a DTD or Schema is used to define the allowed XML vocabulary. This includes things like the tag and attributes names and their possible values.
If you are using XML you should always be thinking bigger than just Arbortext. Even if you *can* make something work in Arbortext, doesn't mean you *should*. You may be setting yourself up for bigger problems down the track if the XML documents you are making are not well-formed and valid.