Messaging or otherwise called "Chat" is the collaborative space of the notebook.
It allows you to perform multiple actions, but let's look here at its main function: exchanging messages.
In a notebook, the messaging is on the left side part:
Exchange messages with members of the book
Share a message directly with all members of the book, simply by clicking on the Send button.
Join an attachement to your messages
By clicking on before sending the message.
Send a message by SMS
Send a message by email
Personalize your messages