Sequence and communication diagrams are other UML diagrams that can model the dynamic behavior of your system. These diagrams focus on showing detailed interactions, such as which objects are involved in an interaction, which methods are invoked, and the sequence of events. Sequence diagrams can be found in Chapter 7. Communication diagrams are covered in Chapter 8.
If you haven't already, it's also worth reading Chapter 2 on use cases because activity diagrams offer a great way of showing a visual representation of a use case's flow.