Relations can be used for a wide range of purposes. That is, relations may cross scene boundaries and reference objects at a different point respectively period of time. Relations having source or destination objects in other scenes are an easy way to reference objects or events that happened in the past or will happen in the future. For an example see Section 4.8.2 . For further discussion of dynamical issues see Section 5.5 .
Relations distinguish between source and destination objects.
That is objects that are the origin of a relation and objects that are the target of a relation. Objects may be both origin and target, i.e. bidirectional relationships are possible. Figure 4.12 shows three simple examples of how to use relations. For ease of illustration a notation with arrows pointing from the origin to the destination objects is being used with the describing objects in the middle. Note that this is just one way to realize relations.