In this book, I have largely avoided discussing the technical details of XML, as opposed to specific issues relating to SVG. However, you might find that envi-sioning the idea of SVG visual components in their XML context is helpful.
Every XML document begins with a top-level, invisible entity—the document entity—that can contain one or many other entities. Those other entities can be contained within the document entity or be stored externally, typically on a computer file system.
You can look on these XML entities as predefined nonvisual components. The important thing to realize is that you can define your own entities and then use them as often as you want in XML documents.
An XML processor ...