When comment is submitted, agent can select comment to be Public or Internal.
Ticket Requester is notified only for Public comments, Internal comments are never sent to Requester (also if ticket Requester is an Agent he will not be notified for Internal comments).
For users specified in ticket CC field:
•Public comments are sent to all CC users.
•Internal comments are sent to CC users if User Type = Agent, not sent to CC users with User Type = End User.
Ticket Assignee is notified for all comments submitted by other user.
Note that a user never get notification for comments submitted by himself.