Use messages for:

• changing the task state: You can change the state of a task using messages. To do this, select the Message Type that will change the task from its current state into the one you need while creating a message. Task states and transitions between them depend on the task workflow determined by the category of the task.
• arranging interaction between users: The Handler task field defines the user whose response is necessary to continue processing the task. If you need another user work on task (for example: you have a code that needs to be tested or you need to ask another developer a question), specify the needed user in the Handler field and add the message.
• monitoring the progress in working on a task: In the process of working on a task, you can add messages describing the current state of affairs and specify the time spent on the task.