You can name classes before you know they exist An interaction diagram captures the behavior of a single use case. Depending upon when you first create the diagram, the objects at the top may be real classes, or just class place-holders. In this case, since the classes already exist, they refer to real classes. But if you constructed the diagram before the classes were created (which often happens), you can name them as seems appropriate, then later update the references to the finalized classes. Note also that an interaction diagram is one path through a use case—in this instance, the "good" path (everything validates successfully). —LJJ