I have spent the last month digging into the use of rel_segs to expand the existing acs-subsite rel_segs in order to provide more granular control over users' actions. My excursion resulted in gaining the knowledge required to expand the default two role (Members and Administrators) acs-subsite organization into a four role (added Guests and Managers) organization. This included a trip through roles, rel types, rel segs, and the associated tcl and db apis.
Finally, I wrote this up into a general Howto with example code and posted it as another thread: https://openacs.org/forums/message-view?message_id=116231
Have a look and let me know if you think it's useful.
Randy
