o, con- sidering that different persons will work with different fragments, it is important that relations between fragments in the fragment structure represent simple sys- tem relations, SO that model configuration management is facilitated. To handle the fragment structure is of primary concern to the model management system, but it is of equal importance that users understand it, which means it must be simple and logical from the point of view of systems architects and designers. Users of models may normally work on one fragment at a time, but they will often need to view more than one fragment simultaneously. For example, to ana- lyse and design a solution to a new, network-wide application, the modeller needs to see the physical structure of the system. Whatever set of fragments or part of a fragment the user sees on the screen is regarded as a view of the sys- In SM&AML, the type of fragments that may exist are primarily defined by the architectural principle integration hierarchy, and cannot be modified by AML- users. Note that the fragment structure of a sysnet model is based on separation of concerns as regards system properties, not product structures, as was discussed in Sect. 1.5.2 (see also Fig. 1.12). This fragment structure fulfils some essential requirements for model management: A fragment that describes a collaboration structure is logically simple. It describes a collaboration structure of units, interconnected by the type of com- munication relation that is characteristic for the fragment type (i.e. the abstrac- tion level). Fragments are therefore easy to manage, and easy to understand and work with. Relations between fragments are also logically simple, which simplifies model consistency management. Relations between fragments represent either some type of communication relation, they may associate different structural levels by substructure relations or they may describe how units are related between levels of the integration hierarchy. Attribute specifications are associated with fragments