Communication diagrams and sequence diagrams are both UML interaction diagrams, which can be transformed into each other, have similar responsibilities, are message-driven, and have sequentiality.
However, there are also obvious differences. Communication diagrams express the cooperation relationship between objects, while sequence diagrams express the time sequence of messages occurring between objects.

Communication diagrams are mainly composed of objects, messages, and links.

Object: The concept of objects in collaboration diagrams and sequence diagrams is the same, but in collaboration diagrams, the creation and destruction of objects cannot be represented, so there are no restrictions on the position of objects in the diagram.

Message: A message consists of a sender, a receiver, and a message name. To indicate the sending order of messages during the interaction process, a sequence number needs to be added to the message.

Link: A link is the connection between objects and is an instance of class association. Links use solid lines or arcs to connect two objects.

Messages are generally divided into synchronous messages, asynchronous messages, return messages, and self-associated messages.

Synchronous Message: After sending a message, the object needs to wait for the receiving object to respond and return the message before proceeding with other work, represented by a "solid line arrow";

Asynchronous Message: After sending a message, the object does not need to wait for the return message from the receiving object and can continue executing other work, represented by a "solid line + greater than sign";

Return Message: Returns from a process call, represented by a "dashed line arrow";

Self-associated Message: Represents a method's self-call or a call from one method to another within the same object, represented by a "semi-closed rectangle + solid line arrow below".

There are two types of message numbering: one is non-hierarchical numbering (sequential numbering), which is simple and intuitive; the other is nested numbering, which is easier to represent the containment relationship of messages.

1. Set the context of the interaction

2. Set the scene of the interaction

3. Set initial characteristics for each object

4. Describe the links along which information may be transmitted between objects

5. Set the sequence number of messages

For Use Cases: Reflects the logical expression of the specific usage context by describing the transmission of messages between objects

For Objects: Shows the spatial organizational structure of objects and their interaction relationships

For Classes: Demonstrates the implementation of a class operation