 that describe collabora- tion structures. There is absolutely no overlap between attribute specifications that belong to different collaboration structures. Fragments that are related by integration relations are backward compatible, which means that if you modify a fragment on a more integrated level (less 1.6 Requirements on a Modelling Language for Systems Modelling abstracted level), it cannot affect a fragment on a less integrated level (more abstracted level). All modelling tools provide some set of predefined views. The language user inputs his model in terms of these views, and he browses the model in these views. To support some editing flexibility, overlap between views is sometimes offered. For example, the tool might offer one view that shows the sequence of events in a sequence diagram and another view that shows the same thing in a collaboration or activity diagram. Redundancy problems are solved by the modelling tool. The parts of a sysnet model that can be viewed are the fragments of the fragment structure (which are non-redundant, as system properties are concerned). In gen- eral, units of collaboration types of fragments can have interfaces in three different logical dimensions: the distribution dimension (D), the communication dimension (C) and the processing dimension (P). Together, these dimensions define a func- tional space. Interfaces in different logical dimensions are kept separate in the sys- net model (and are, to some extent, also represented by different graphical symbols). This provides an intelligent viewing tool to show the sysnet model (or more likely, a part of it) in one, two or all three dimensions by suppressing infor- mation and by linking model fragments. Considering all possibilities, there are, in all, seven views to consider for tool designers: DPC, DP, DC, PC, D, P and C. The selection of views is controlled by tool users. Past experience in sysnet modelling has shown that an important aspect of view- ing sysnet models is the ability to view several fragments in an ove