d shape, denoting the general entity type functional element, which is any entity that shows a pattern of behavior). 1.2 Added-Value versus Common-Value Languages These entities are derived nodes (The relation is a network interface) These entities are logical nodes (The relation is a link route) These entities are layer elements (The relation is a message protocol) The left entity is a relaying actor, the right an agent (The relations are two layer interfaces of type CSP, an actor-agent interface of type SAC, and a message protocol) These entities are all logical nodes (The relation between B and C is a substructure relation. The model tells that A and B are parts of C and that only B connects to external nodes) Fig. 1.4 In AMLn, most entity types are defined indirectly by the relations they terminate The actual type of entity is defined by the relation types that it terminates. The number of predefined relations increases of course. There are nine basic relation symbols in AMLn. Some symbol specialization possibilities exist, which increases the total number of specific symbols to around 20. Since an AMLn model consists of several model types (or views), however, only a few relation types are used in each model type. 1.2.2 AMLn versus UML The author has been told that AMLn takes a controversial path to systems mode- ling because it does not rely on UML. Proponents of this opinion do not understand that there cannot be any controversy since UML and AMLn are used for modeling completely different things. AMLn is an added-value language, used in early development phases. UML, a common-value language, is used first when a supplier wants to realize an AMLn model. An AMLn model describes a network system in a way that is independent of how it will be realized (sometimes denoted as its "architecture"), i.e., the entities and interfaces of the model are undefined as regards their realization. A UML model describes a system of software elements and interfaces. Such a system is itself a component that realizes a part of 