The private reply is a feature from the support forum which lets you hide your reply from public view and is only visible to you and the support staff.
You can use this feature if you wish to hide certain information from the public view and wish to share it only with the support staff.
This feature is useful if you wish to send credentials to support staff, or if you simply do not want to share your site link with anyone other than support staff.
Using Private Reply
Simply check on the Set as private reply and Submit the reply to use the private reply feature. You will see the Private badge as an indicator that your reply is private.
Multiple Users
Since other users can participate by replying to the thread, those participants can also mark their replies as private. In this case, the topic author can not see their reply, but support staff still do.
But if the support staff marks the reply as private, only the topic author may view the reply and not the participants.
Private Topic
You can not mark the topic as private. This feature is only available for reply. If you wish to create a private topic, you can create a topic and reply to that topic with the private feature instead.