Many elements model classifications (such as Classes and Actors), whilst other elements model instances of such classifications (such as Objects, Actors again, and Sequence diagram objects). These instance 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.
You can define a classifier first, and then instances of that classifier. Alternatively, as a model develops from a rough sketch to a detailed design, many objects become examples of a defined Class, 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 element (its classifier), such as a Class. 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 you describe a Person Class with attributes such as Age, Name, Address and Sex, and functions such as GetAge and GetName, then when you associate your 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).
This is a powerful means of moving your model from the analysis phase into detailed design.
Elements that are classifiers and support instances of themselves at runtime can be dropped from the Project Browser as a link to the classifier itself, or a new instance of the classifier. The example below shows a linked Node element on the left and an instance of the Node on the right. Note that the instance is drawn like a simple element with the : <ElementName> displayed. If you name your instance it displays <InstanceName> : <ElementName>.