Many elements in UML model instances of classes, such as objects, actors and Sequence diagram objects. These elements represent real things in a run-time scenario; for example, a Person element named Joe Smith. In UML this is written as Joe Smith : Person.
As the model develops from a rough sketch to a detailed design, many objects become examples of defined classes, so in the early analysis phase you might model a Joe Smith and a Jane Smith, and later a Person class from which Joe and Jane are instantiated.
Enterprise Architect enables you to associate an object with its template or class (its classifier). Doing this greatly increases the descriptive power of the model in capturing the functionality and responsibility of objects at run-time and their associated state. For example, if we describe a Person class with attributes such as Age, Name, Address and Sex, and functions such as GetAge and GetName, then when we associate our object with the Person class it is seen to have all the Person class behavior and state (as well as inherited state and behavior from Person's ancestors).
Tip: This is a powerful means of moving your model from the analysis phase into detailed design.