Architecting and designing : UML 2.5 : Model Elements : Association Node : AssociationNode (Invisible)
  
AssociationNode (Invisible)
An Association classifies a set of tuples representing links between typed instances.
To create an Association between two Model Elements or Classifiers, you create an AssociationNode and then relate the AssociationNode to each Model Element or Classifier through an AssociationNode to Class as Property relationship.
When the relationship is created, the Association Node (Invisible) will be made invisible so that only a solid line will show.
See also
Association
AssociationClass
CommunicationPath
Extension
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Association Node