UML Modeling Techniques > Class modeling > Associations and io flows > Overview of associations
Overview of associations
An association expresses the relationship between Classes, Data Types, Interfaces, Signals, Actors, Parts and Ports.
On a Class Diagram or Composite Structure Diagram, use the appropriate toolbar button to create an Association between two items on a diagram.
Associations can own IO Flows, which you create through Class Diagrams or Composite Structure Diagrams.
When a Class Diagram shows an association or composite aggregation symbol starting from an Activity, that link symbol represents a Call Behavior Action, Central Buffer, Data Store, Input Pin or Output Pin, rather than an Association dictionary item. See Creating a composite aggregation between two activities on a class diagram, Showing an activity call tree on a class diagram, and Showing an association between an activity and a class on a class diagram.
On a Composite Structure Diagram, unless you have a good reason to create Deep Associations, create Associations on your Composite Structure Diagrams as Shallow Associations. Creating Associations as Shallow Associations makes it easier to use those Associations in other contexts without creating redefinitions. For more information about shallow and deep Associations, see Shallow and deep associations.
An Aggregation is an Association item that has its Aggregation property set to 'part of' or 'made up of'.
A Role is one end of an Association. A binary association has a Role at each end. Roles can be identified by an optional name, otherwise the names of the connecting items are used as the Role names.
Role names are similar to Association direction names except that a noun rather than a verb is used. Role names can be used to uniquely identify one end of an association, and to distinguish between two associations linking the same items.
A qualified Association relates two objects and a Qualifier. A Qualifier is a special Attribute that reduces the multiplicity of an Association. One-to-many and many-to-many Associations can be qualified. The qualifier serves to distinguish the set of items at the many end of an Association. A qualified Association can be considered a form of Ternary Association. Qualification will usually reduce the multiplicity of an Association from many to one.