HP-Helptag-SGML is a SGML-based hypertext help-system for commercial Unices (HP-UX, Sun Solaris, IBM AIX).
More information:
Common Desktop Environment (CDE) Help System: http://www.oasis-open.org/cover/gen-apps.html#cde (general overview, with some more links)
"Common Desktop Environment: Help System Author's and Programmer's Guide": http://h71000.www7.hp.com/doc/73final/helpguide/helpguide_5.html.