This mechanism would be used where new elements need to be added to the standard schema. An example might be a requirement to represent vehicles through a new Vehicle element. Under these circumstances, the author designing the extension needs to provide an associated XSD schema file defining the syntax of those new elements. The new XML elements and attributes are then referenced with the prefix of the associated namespace. For instance:
Text with embedded Vehicle reference <vh:VehRef Key=’Harley1’/>
A schema validation operation on such XML would use both the standard schema associated with the default unnamed namespace and the extended schema associated with the additional namespace.
STEMMA implements a number of subject entities — Person, Place, and Group — and it is this area where schema extensions are most likely to be proposed. The existing entities are treated uniformly using OOP techniques such that they all inherit from a base subject-entity class. They could be categorised further to differentiate inanimate and animate in order to represent Animal as distinct from Person but where both share a common type of hierarchy (i.e. lineage). This would enable, for instance, pedigree dogs to be represented along with their breeders, and the relationship of the dogs, breeders, and judges, to their shared events to be described. Although the Group is a concept independent of people, only Person entities can currently be associated with it. If an Animal entity were defined then it is envisaged that both Person and Animal could be associated with the same Group; an example being a regimental mascot.