ry different methods, depicted in Fig. 2.21. Note, however, that in interface units of implemented systems, the two meth- ods are often mixed. Substructure, defined Substructure, defined by translation by discrimination part of M' set M'-set M-set {Did,d(M')} part of M'_set Fig. 2.21 Two methods for creating substructures: by translation and by discrimination; d(M') means "the data of an M'-message" In the translation approach, external messages or events are terminated in the IU. Thus, internal messages and events have no one-to-one correspondence to external messages and events. The IU uses the functions of other internal units to provide the proper response in the external interface. The IU is, in this case, said to be an agent (A). In the discrimination approach, messages or events that are defined by the exter- nal protocol are terminated in different internal units. The IU does not know what the messages are about, with one exception: there is at least one parameter in every M'-se message that guides the IU as to which internal unit should deal with the message or event. The generic name of such a parameter is discrimina- tion identifier (Did). The IU is, in this case, a discriminator (D) that performs discrimination. The data d(M') are not interpreted or acted upon by the discrim- inator. They are instead relayed to other internal units. A substructure often depends on factors that have very little to do with which messages or events are defined in the external interface. To a large extent, it depends on where resources are allocated in the internal structure and on the struc- ture of the node platform, on which the elements rely. Thus, in general, there is no one-to-one correspondence between what happens externally and internally. This is the case when the interface unit is an agent. Figure 2.22 shows an example. 2 Foundations of Sysnet Modelling Substructure, defined Transaction specification by translation for external message type M1 in: M1 Transaction out: m1 specification for M1 in: m2 in: m3 out: m