A fantastic visual representation of the key relationships in Connection database schemas by Mark Myers.  None of this is documented by IBM publicly so this is entirely Mark’s effort to take apart and document.  Some of us have tried it in pieces but this is by far the most comprehensive and useful attempt to document the underlying architecture I’ve seen.

Another one for the bookmarks…

http://www.stickfight.co.uk/blog/Connections-Db-Schema-Tip2-Finding-the-UserID