<signal-spec>
The definition of a signal between two parts of the same level or between a part and a part type is always contingent upon the signal in the higher ranking part type. As a rule, signals do not have a direction. This is determined by the respective port definition (in, out, bi).

Caution
Strictly speaking, no signals can be fed extraneously to systems and part types. This only becomes possible when a system or a part type is mounted and employed ( "instantiated" )in a higher ranking system or in a higher ranking part type and therefore becomes a part.

The following figure illustrates the situation:
Figure 6: An example of interfaces and signals
The signal at "1" connects the control unit part to itself. The signals at "2" connect the parts control unit and hydraulic aggregate to one another. The signal at "3" connects the hydraulic aggregate part to the ABS system, while the signal at "4" is fed to the ABS system extraneously, and therefore can only be specified if the system is instantiated.
The signals at "1", "2" and "3" in the last figure are described according to part type ABS under the point <architecture>/<signals>.
The signal at "5" is an internal signal of the control unit. This signal has to be specified within the part type of the control unit.