The object diagram represents instances of class or interface, the sequence diagram represents the dynamic behavior of a class or interface, and the communication diagram represents those instances in a communication mode. For all these reasons, you can link a class or an interface to an object in an OOM.
You can drag a class or interface node from the Browser and drop it into the sequence, communication or object diagrams. You can also copy a class or interface and paste it, or paste it as shortcut, into these diagrams. This automatically creates an object, instance of the class or of the interface.