[A] Comment is feature you can use to comment about that particular transaction within the team and outside the team (a counterparty).
[B] You can also choose the visibility of your comment to be "Internal" or "External". Internal comment can be used to notify your team member, just add @ and type the name of the user you wish to notify.
[C] Select then from the list, write down your message and click on Submit. You can also upload files. That user will receive a mail mentioning your message.
[D] In the same way you can use External comment to communicate with a counterparty.