The basic concept is straightforward. If any VATSIM ATC member goes online, and finds that there is no ATC coverage in the positions immediately below his, he will automatically cover these vacant positions whenever possible (depending on workload).
The "normal" ATC structure in VATSIM is shown on the right of the coloured box below, using our fictitous airfield as an example. The top level is Control (called Center in the US) abbreviated to CTR. The bottom level is Delivery. Note that less busy airfields might not use the Delivery position, giving Delivery responsibility to the Ground controller instead.
So for example if an ATC officer takes control of the Approach position at an airfield, and there is no other positions being manned at that airfield, he will control not only the Approach position but also the Tower position immediately below him, as well as Ground below that - and Delivery too, if the airfield uses that position.
Note that the member controlling Approach will not also control Area Control, because that position is above his, and this is a Top-Down system!
The following animation shows the top-down system in action. The map shows an island with an airfield called “Island Airport” on it, with the ICAO code "ISLE," and all its positions can be manned: Approach, Tower, Ground and Delivery. There is also the Area Control position above the others, covering Island Airport and probably several other airfields too, which provides an en-route ATC service. It's called “Mainland Control” in our animation.
Remember that Area Control is called Center in North America, so Mainland Control would be called Mainland Center there. In either case, it's code is MAIN_CTR.
The list of our ATC positions, in the table to the left of our map, also shows an entry for ATIS: the Automatic Terminal Information System. This is not actually a control position but it's often shown on the maps used by pilots when planning a flight, so it's included in our list and on our map.
You can switch on and off ATC positions using the on/off buttons in the table below. When you switch a position ON, you'll see what service is avaliable from that controller, and also see who to contact. The map on the right will also change to give you graphical informtion about ATC coverage.
Any and all positions can be switched on and off, and the diagram will change each time to show you the result. The animation starts with no ATC positions manned, so all the buttons are Off.
Once you have played around a bit and seen the effects, you can start to test yourself. See if you can guess what might happen each time you make a switch change. Go nuts!
Regarding Approach, Tower, Ground and Delivery, the top-down system is restricted to the airfield you are positioned at. So if you’re at the gate at Airfield A and these four positions are all unmanned at your airfield, you can’t use the ATC service available at Airfield B, even if it is close to you. But if the Control (Center) ATC in your area is online, you can get ATC service from them.
But what happens if only Tower is on, and you leave Tower’s area of control? Who ya gonna call? The answer is UNICOM, on 122.8. You met UNICOM in Part 2 of the New Member Orientation Course.
In case you missed it in the animation, there is one other thing to note about who to call. If Ground is manned “from above” by another ATC position, say Approach, and you want Ground service, you don’t call on the Ground frequency: you must call Approach on their frequency for the service which Ground would provide, eg. to request taxi clearance you would tune the frequency for Approach, and say, "Approach, [your callsign] request taxi." You wouldn't tune the normal Ground frequency and you wouldn't call them "Ground," you would call them "Approach" because that is who they are.