Relationships in Use Case Diagrams

• This first, generalization, has already been discussed in the context of Actors. Usually, the use of the generalization relationship is restricted to actors. A generalization is denoted by a solid line with an open arrow at one end.
  
  
• The second relationship is the association. Associations are typically used between actors and use cases to denote an actor performing actions in a particular use case. An association is denoted by a solid line, such as the association in Figure 1 from the Customer actor to the Buy Part use case. Occasionally, you may see an open arrow on an association. Such an association is said to be "navigable", in otherwords messages only go one way. This is very rare in a use case diagram.
  
  
• The third relationship is a dependency. Dependencies are typically used to relate two use cases. It is rare that dependencies are not stereotyped as illustrated in figure 1. Dependencies are denoted by a dashed line with an open arrow.
  
  

There is a fourth kind of relationship for realizations, this releationship is discussed on its own in another section.
 

Figure 1 - Use Case Relationships