Modality
The modality is 1 if an occurrence of the relationship is mandatory. The modality of a relationship is 0 if there is no explicit requirement for the relationship to occur to the relationship is optional. To describe it suppose software which is used through a local telephone organization to procedure requests for area service. The customer indicates that there is a problem. Moreover if the problem is complex, multiple repair actions might be needed. If the problem is diagnosed as relatively simple a single repair action occurs.
Which is a single customer can be gives with 0 or many repair actions.
On the relationship connection closest to the data object rectangles specify cardinality. The vertical bar indicates 1 and the 3-pronged fork indicates many. The Modality is indicated through the symbols which are further away from the data object rectangles. The 2nd vertical bar on the left indicated that there must be a customer of r a repair action to occur. In the circle on the right indicates that there may be no repair action needs for the type of problem reported through the customer.