\hypertarget{extension}{}\subsection{extension}\label{extension}
The term 'extension' as used in XML schema is not used in its formal, mathematical sense. Rather than defining a superset of valid instances, an extension describes a different, possibly intersecting set of instances that structurally extend the base type. In many cases an extension will describe a valid sub-class, but there are cases where this does not hold. In the valid cases we assert a sub\-Class\-Of relationship. If we use extensions simply to add new elements and attributes to an existing type we have nothing to worry about. The problems arise by adding additional occurrences of existing elements.

\begin{itemize}
\item \hyperlink{extensioncomplexcontent}{Extension of Complex Content} \end{itemize}
