We repeat the same technique with the XML source file (source.xml) as we did with the style sheet: we separate the content from the presentation in this file, and obtain an XML description of the layout for this particular page, separate from the description of the content. This mirrors our separation of the general layout of the site from the logic.
This kind of multi-level layout is very convenient when designing portal sites, where there is an aggregation of data from different sources in order to create the "body" part of the page. Using the main document source file as a layout allows us to keep most of the (normally dynamic) data in separate files or data sources where they belong.