 are different for different network types. Most attribute specifications exist on the level of service networks, where most of the functions and behaviour of the sysnet is described. There are basically four types of relations to manage in a sysnet model: Collaboration types of fragments show units that collaborate over communica- tion relations. "Unit" and "communication relations" are generic terms. Each type of model fragment defines its own type of units and communication rela- tion. For example, a physical network shows physical nodes that communicate over network interfaces. A service network shows service objects that commu- nicate over service relations. Collaboration types of fragments associate attribute specifications by attribute relations. Within each set of collaboration type of fragments, individual fragments are related either by the type of communication relations for that type of model frag- ment, or by substructure relations. In the former case, this is because a larger fragment is partitioned into parts. In the latter case, this is because a substructure of a unit has been developed. Collaboration types of fragments of different types are related by integrates relations. Such a relation defines how a unit in a model fragment on a lower abstraction level integrates units on a higher abstraction level. Such integration always requires the specification of some type of integration mechanism. For example, several protocol elements can be allocated in a logical node by adding a discrimination function. 1.5.3 Fragmenting Implemented System Models In implemented system models, many other types of relations may be repre- sented by IE relations, e.g. how software-hardware interfaces are implemented, or that two or more entities belong to a subsystem that is managed in some activity (e.g. system provisioning) as a single entity. In order to manage implemented system models, the information that describes software and hardware products of implemented system are commonly aggregated into sets that also ar