You can organize sequence diagrams to better represent a system.
You can reorder messages, delay asynchronous messages, as well as manage combined
fragments and lifelines.
Reordering lifelines and messages in sequence diagrams
In sequence diagrams, you can reorder lifelines or move a message
across other messages and interaction fragments on the same lifeline or on
another lifeline to reorder the sequence in which the messages occur. After
you reorder a message, the message number automatically indicates the updated
position on the lifeline.