Reference vs. Inclusion
The CDF working group has defined two methods for combination of XML documents, combination by reference and combination by inclusion. What's the difference? Combination by reference is like referencing an image or any other object (actually it leverages the <object> tag) to combine content. Combination by inclusion involves actual mixing of XML vocabularies in the same document. For various reasons, the group has chosen to focus on the first, combination by reference, for the first release of their specification, and for the combination of XHTML and SVG. This approach leverages the already-existing <object> tag to embed SVG content, and specifies the semantics used with that tag.
Combination by inclusion is a different ball game. The next work of the CDF working group will focus on combination of XML vocabularies by inclusion – that is, mixing tags from different XML namespaces in the same document.