ation are defined by AML. AML provides a set of predefined services that can be used to model integration mechanisms. Such services are denoted by service relation symbols that differ from those service relations specified by the modeller. Models of the internal structure of units comprise both local and global (or "remote") communication relations. The former are always denoted by solid lines, the latter by dotted or dashed lines. Note, however, that the concept of "local" and "global" (or "remote") depends on the structural level of the model, i.e. a relation that is regarded as local on one structural level may be regarded as global on a lower level. Basically, AML does not define the symbols used for different types of units (physical node, logical node, service object, etc.). In order to allow for the crea- tion of expressive models, unit-symbols should be definable by the AML user, in particular for physical networks and for resource modelling. This principle con- stitutes a requirement on tool makers to provide features for user definition of unit symbols. The only requirement on unit symbols is that the symbol must show whether the actual unit is specified as a part of the actual model fragment, or is just a way to refer to a unit specified in another model fragment. In the former case, the unit symbol shall have a closed boundary; in the latter case, it shall have an open boundary, as in Fig. 2.60. These units are specified as These units are specified as parts of other model parts of this model fragment fragments Fig. 2.60 Open and closed unit symbols in a model fragment Most model fragment types can show units either as permanent (persistent) units in the part of the sysnet that is modelled, or as temporary (non-persistent) units. 2 Foundations of Sysnet Modelling We say that a unit may have a permanent existence or a temporary existence in the model. The need for distinguishing between permanent and temporary exist- ence of units in models was not invented in AML. Selic et al. (1994) make such a disti