When you draw a relation line between two entities on a data structure diagram, the Associative Data dialog box opens so you can specify the cardinality and optionality of the relationship.
The buttons are used to indicate the cardinality and optionality of the relationship between the two entities. In the picture above, the Customer must place many Orders. Therefore, if there is an Order record, there must be a matching Customer record. This relationship is one-to-many.
The buttons have the options must/may and one/many. Other data modeling techniques allow a richer possibility of cardinality, such as one, zero or one, zero, one or many, many, and so on.
The Order must be placed by one Customer. The reverse phrase is placed at the target entity end of the line. It serves to clarify the drawing, but is not otherwise important.
Exclusion arcs are used to indicate complete and incomplete categorization of the relationships. For example, the business relationship expressed by:
"The customer may place an order, or the customer may request a catalog"
would use an exclusion arc to indicate that the relationship is either between customer and order, or between customer and catalog, but not both.
