In this step we shall define the methods of the classes that were discovered during the previous step. In this step we define the behaviour of our system. In other words, we go for the following targets [Rosenberg, 1999]:

1.    Distribution of behaviour to boundary, control and entity objects.

2.    Identification of the interactions between the objects of each use case

3.     Complete the distribution of the methods to the classes.

The methodology that we follow for the creation of sequence diagrams is summarized as follows:

1. We copy the text of the corresponding use case to the left side of the sequence diagram.
2. We add the entity objects (from the robustness diagram)
3. We add the boundary objects,
4. Finally, we add the control objects as methods of the respective (boundary and entity) objects. (Note: A control object may correspond to many methods in the sequence diagram).
5. Additionally, we can create collaboration and/or activity diagrams to elaborate further.

The fourth step is the most difficult one and we should make a decision as to which methods correspond to which objects.