The HL7 control query committee on its September 1998 meeting in San Diego set up a task force to work out a redesign proposal for HL7 version 3 data types.
After over 5 years of intense work in designing the content, balloting and reconciling, in early 2004 the membership ballot finally passed and the specification is due for issuing as an ANSI specification.
Extensive notes have been taken during the early design phase that may still be of interest and are preserved here.
The final documents should be requested through Health Level-7, Inc. Earlier and current draft versions may still be available at this site.
See a brief introductory tutorial here.