 View can contain only one TwoParty- Connection". The resulting model could then be redrawn as in Fig. 1.20. Guess which of the models was most appreciated! Logical objects: Physical Active, Passive, Passive, channel, Switching Access breaks breaks does not reserved Switch Switch Confer- Two-party speech speech break Logical -ence connection speech path connected channel Fig. 1.20 A model with high expressive values This example indicates both the importance of overlay techniques and that the users of domain-specific modelling languages can choose their own symbols. In order to not force users to invent symbols, AML provides a neutral unit symbol (a rectangle), but pre-defines all symbols for relations. Since each model fragment type has its own types of relations, the relation symbol thereby implicitly defines 1.6 Requirements on a Modelling Language for Systems Modelling the type of units. It is instead assumed that the editing tool allows users to exchange these unit symbols for any type of symbols they want, in order to increase the expressiveness of the model. Expressiveness is a subjective quality that depends on who interprets the model. As indicated previously, an incomplete version of a sysnet model may be created in early phases that will in later phases be completed and refined with details. In early phases, the model is primarily interpreted by human users. If the model also acts as requirements for product design, it should also be machine interpretable, since we want to map the model on some more general language, e.g. on a modelling lan- guage for software or directly on some programming language. AML considers, through different notations, the need for distinguishing repre- sentations for humans from representations for interpretation by machines. To translate a model in human representation to a machine-interpretable notation is called specification refinement. Specification refinement implies not just another choice of representation, but also addition of extra details to make a specification co