On Communication Diagrams, the Messages are decorated by a small arrow in the direction of the Message close to the
Message name and sequence number along the line between the lifelines

The sequence numbers show how the other messages are
sequenced. 1b.1 follows after 1b and 1b.1.1 thereafter etc. 2 follows after 1a and 1b.

Sequence expression

The sequence-expression is a dot-separated list of sequence-terms followed by a colon (':').

sequence-term '.' . . . ':'

Each term represents a level of procedural nesting within the overall interaction. If all the control is concurrent, then
nesting does not occur. Each sequence-term has the following syntax:

[ integer | name ] [ recurrence ]

The integer represents the sequential order of the Message within the next higher level of procedural calling. Messages
that differ in one integer term are sequentially related at that level of nesting. Example: Message 3.1.4 follows Message
3.1.3 within activation 3.1. The name represents a concurrent thread of control. Messages that differ in the final name are
concurrent at that level of nesting. Example: Message 3.1a and Message 3.1b are concurrent within activation 3.1. All
threads of control are equal within the nesting depth.
The recurrence represents conditional or iterative execution. This represents zero or more Messages that are executed
depending on the conditions involved...