There are the main estimates, and in the last few years we've had supplementary estimates (A), (B), and (C). If you went back further you would see that in some years there were only two, so it was (A) and (B).
Sorry, I may have misunderstood the question, parts I, II and III of the estimates....
I'll just finish my thought on the supplementary estimates. That's largely a function of the urgency of departmental spending. If you're curious about the actual content of the main estimates themselves, part I is the government overview. It's quite useful in setting the context for what's in the main estimates document. Part II is by department and is quite useful in looking at changes for the department itself. Part III is on the reports on plans and priorities and the departmental performance reports.
That's why we say there are three parts to the estimates. But I'm not clear if your question was on that part or the fact that we have supplementary estimates (A), (B), and (C) as well. I'm not sure if I answered your question.