Sequence Numbers

Sequence numbers can be assigned to messages in both communication and sequence diagrams, but they are most important in communication diagrams.

When you create a message in a communication diagram, the default value of the sequence number is calculated with regards to the most recently created or modified message. The first sequence number created is 1.

A succession of sequence numbers is built from the most recent sequence number plus 1. For example, 3 + 1 => 4, or 2.1 + 1 => 2.2

The creation of sequence numbers respects the syntax of numbers already used in the diagram (1, 2, 3, etc... or 1.1, 1.2, etc.).

By convention, the addition of letters to sequence numbers signifies that the messages are parallel. For example, the messages with sequence numbers 3.1a and 3.1b are sent at the same time.

If you need to change sequence numbers manually, you can move or insert messages in the diagram or increase and decrease the sequence numbers.

• Moving Sequence Numbers
  You can move and insert message numbers within the communication diagram.
• Inserting Sequence Numbers
  When you insert a new message with a new sequence number in an existing communication diagram, the sequence numbers are recalculated with respect to the following rule: for each number after the insertion, all numbers are incremented by 1.
• Increasing Sequence Numbers in a Communication Diagram
  You can increment a sequence number using the following methods:
• Decreasing Sequence Numbers in a Communication Diagram
  You can decrease a sequence number using the following methods: