Recently someone asked me what was the real meaning of the ‘Cid’ field which appears in the output of commands such as !process and !thread in the kernel debugger (kd). Though from a practical perspective I was aware that these represent the Process ID and Thread ID, I was unsure of what Cid stands for. In course of a search, I found a public source which answers the question. Cid is short for CLIENT_ID which in turn is an undocumented structure.
The public source is a free PDF version of the excellent “Undocumented Windows 2000 Secrets: A Programmer's Cookbook” book, which you can now find at http://undocumented.rawol.com/. I think this resource is a very useful one for all those interested in Windows Internals and debugging as well. Go take a look at it!