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 subClassOf 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.

• Extension of Complex Content

---