Imagine a group of 5 individuals. Lets call them The Committee. Their goal is to overthrow the US Government.
The Committee hire a specialist, whom we will call Control. The Committee draft a series of objectives for Control to accomplish: Carry out a major terror campaign against US interests. The campaign must have significant economic effects. It must create a climate in which civil liberties can be eroded.
Control has no idea what The Committee's goal is. It could be to bring down a major corporation, to swing an election, to get a certain piece of legislation through congress, or it could be to overthrow the USG. Control has no interest in finding out and doesn't spend much time worrying about it.
Now picture a number of branches radiating out from the center where Control is located. Each of those branches is thick at first and runs to twigs at its end. The head of one branch knows that the goal is to crash aircraft into the WTC, Pentagon and White House. The head of another branch knows that the goal is to embezzle money from charities to secretly fund martydom operations, perhaps in the Middle-East. The head of a third branch knows that the goal is to cripple the US postal service. And so on. One branch is very short and thick. We can call it The Cleaners. More on them later.
None of the heads of branches know that there are other branches, and none of them know that they are part of something larger. Why should they? Control knows the job. Meanwhile The Committee know nothing of the means Control is using to accomplish the mission.
Moving down the branches, the head of a twig knows that the goal is to make a few thousand dollars by providing housing for 5 brothers for three months, quietly. This twig might be a Dutch widow with a heroin habit and a few empty bedrooms. Or it might be a Midwestern property owner with an expensive daughter. Why should the twig care where the money is coming from? Why should the twig suspect that it is lying at the end of a large branch connected to a center? The twig rents rooms.
Another twig might know that the goal is to highjack an American airliner out of Boston to secure the liberation of Political prisoners in Israel. Yet another twig might know that the goal is to warehouse an old friend's milling equipement in her basement while his business moves from one location to another. Perhaps there is a slip here, perhaps she dies of inhaled Anthrax. There are many such slips at the level of the twigs, but who would think of connecting them?
In this system, information moves downstream, always. Never upstream. Control gets information from the Committee, and control gives information to the heads of branches. Control certainly has no need or desire to find out who The Committee might be.
In such a long-range campaign (certainly years, perhaps decades), The Committee has no idea what the operations are, and certainly doesn't want to find out ahead of time. But The Committee must know which events are part of the campaign, because they have their own moves to carry out in the wake of these events. Yet, for security reasons, Control can't initiate communication with The Committee.
But any yahoo can blow up a bus or a tube in a large capital. In fact, the Committee is fervently hoping that Control's activities will be emulated. It can only help them. The Committee has specified an open code that Control is to use to reveal his operations. A code that will be self-evident to those who know it, but largely invisible to those who don't.
There is another security measure. The dangerous operatives, those that could easily guess what they are a part of (or those that have), must be eliminated. Hence the Cleaners. The Cleaners know absolutely nothing. They get a name, photo, and travel advance. Control probably uses a different cleaner every time, and avoids recruiting them from the same source.
How large is this conspiracy to overthrow the USG? Five people who know what the goal is, but have no idea what the means will be. One person who knows what the means will be, but has no idea what the goal is. Hundreds, perhaps thousands of individuals who will never have any idea that they are part of it.
Control might deal directly with a total of 8 or 9 people, all under false pretense, and most under false flags. So 15 people might have knowledge that might be recognized as dangerous by someone who knows the whole plan. But no one knows the whole plan. The rest is a constellation of unconnected dots.