This paper provides a classification of the different functions and the paper gives a more precise definition of control actions that are relevant to air traffic control systems. In particular, the overall problem is viewed as a typical large scale complex problem and a multilayer representation of the different control functions is provided by using illustrations for each layer of the specified objective, the time horizon and the links within the overall system. At the same time, potential benefits and present limitations of new control methodologies are discussed and trends for future research are indicated.