The paper is the first in a series. I will get the others and have a look in a day or so. I am a bit busy at present. This paper starts off with some history and some examples of how it used to be done with a table, a trigger and a view. This is an excellent example that talks about performance issues and the appearance of sys_context() and its displacement of userenv().
Jonathan then goes on to detail contexts and gives examples showing how the previous example can be improved. He talks about and explains contexts in detail. This is an excellent article. I will track down the other parts and get back to you on those soon as well.