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.