Extend Enumerated Lists in XML Schema
http://www.ibm.com/developerworks/library/x-extenum/

Kiel, W. Paul
IBM
2008

Abstract:

The addition of new values to a list is a common and necessary requirement. Schema designers often seek to build into the architecture a means to permit additional values that were unknown at design time. How can schema designers create an enumerated value list that is extensible and easy to implement? Discover several approaches used to achieve this goal.