ElementKey

Element key, for schema content or resources. It must always contain a string ID, but typically contains more data to make it sufficiently unique.

It does not contain any URI, to make the elements having those keys as portable as possible.

It is assumed that only the 2 sub-types SchemaContentElementKey and ResourceKey are needed, that is, that each element (to refer to in a LocatorNode) for which there is no specific LocatorNode sub-type is either schema content or a ResourceNode (or its content).