An n-Ary Association element is used to model complex relationships between three or more elements, typically in a Class diagram. It is not a commonly-employed device, but can be used to good effect where there is a dependant relationship between several elements. It is generally used with the Association connector, but the relationships can include other types of connector.
In this example there is a relationship between a Company, an Employee and a Salary.
OMG UML Specification:
The OMG Unified Modeling Language specification, (v2.5.1, p.201) states:
Any Association may be drawn as a diamond (larger than a terminator on a line) with a solid line for each Association memberEnd connecting the diamond to the Classifier that is the end’s type. An Association with more than two ends can only be drawn this way.