should provide efficient access to the that represent with structurally significant within the terminology. lists the that have the most clear-cut structurally significant . A should enable access to these by an easy to use name of enumeration. In addition a should provide a service that rapidly determines whether a given is a of any of these . It is also useful to for the to extend similar functionality to all direct of the ( ) and to of .

Table 30. Essential concept Identifiers





The . All other are of this .

All active metadata are of this .

All enumerated values applicable to core are of this .

All and all related metadata are of this .

All are of this .

The used to specify the between .

All ( ) are of this .

With the exception of the (see above) all that are used in the are of this .

Each of this represents an extension namespaces allocated by the .

of this to provide nodes in . They act as grouper categories that do not have any semantic meaning and thus do not appear elsewhere in the hierarchy.