Below are the different symbols used in the previous example.

Object

The object rectangle represents an object in the communication diagram.

Message

The message includes the interaction between two objects at a given time which is represented by a number.

Arrow

The arrow represents the direction of the message which leads to the next object.

Links

The links represent which objects can send and recieve messages from one another.

Link Loop

The link loop occurs when an object sends and recieves the same message.