From the manual
This program is not a game, but sets out to realistically simulate air traffic control, which is a subject touched on little in the everyday world. Whilst most people who have flown, whether for business or pleasure, appreciate that the aircraft in which they have travelled has been "controlled" in some way, few actually realise the complexity of the system which ensures their safety. And of those who do know what the initials ATC stand for, not many appreciate that most of the work involves controlling aeroplanes as they fly between airports, not just as they approach places such as Heathrow and Gatwick.
The main aim of Air Traffic Control is to ensure a safe and expeditious flow of air-traffic; safe in that all aircraft must be kept apart, and expeditious so as to avoid undue delay. To facilitate this, airways, so-called "motorways in the sky", have been established linking the major airports of the world. lt is within these airways, and the Terminal Areas which surrounded the airports themselves, that most of the air traffic now flies, safely controlled by ATC.
It is the aim of this program then to simulate this Area Control of Air Traffic. The actual area used is the Hurn sector of the London Air Traffic Control Centre, centred roughly on the Bournemouth/Southampton area - encompassing most of the air traffic from the south and midlands of England going to and from Spain, Portugal, parts of France, Africa and the United States.
You must climb and descend the aircraft according to the rules set out in this booklet, guiding them through your airspace so that when they leave they are going in the right direction, at the correct height, and safely separated from each other.