1. A method for implementing a data system providing computer-controlled services and comprising a first part (12) comprising elements representing the real world, and a controlling part (11) controlling the first part, the method comprising the steps of dividing the controlling part (11) of the system into an unchanging part (14) and changing parts (MO) which model, within the controlling part, the elements representing the real world and communicate with them to provide the user with services, providing the part (12) of the system representing the real world with elements of several different types and several different practical implementations of at least one type, so that a certain changing part corresponds to one or several practical implementations and communicates with the corresponding one or several implementations, implementing a separate element in the controlling part (11) of the system, the unchanging part (14) searching from said separate element the concrete implementation of the changing part that corresponds to the desired implementation of the real world, characterized in that the method further comprises the step of realizing said separate element in the form of a selector part (61) that is common to the changing parts and that selects the concrete implementation to be used on the basis of the parameters supplied thereto.