The module will be available for Odoo 9.0 soon. You have time to buy Internal Messaging before price increase! (When you purchase 8.0 version, you get a right to download 9.0 and any further module versions).

By default, to send a private message to specific recipient(s) you need to delete other followers included to the same document. The module allows to send private messages to recipients you specified, regardless of who are in the followers list.

It simplify internal communication in leads, when you need to send some private messages to your colleagues before reply to customer. It helps in records like project tasks too. In a task with many participants, you can easily send message and only specified colleagues will be notified. It allows to have clean inbox for everybody. It's very essential, because people often neglect important message in Inbox full of useless messages.

As a result you can see the message with a dark background, which is sent to the corresponding recipient(s). Other followers of the document will not receive your message.