If you are the moderator for a challenge, you have access to a comment areas that can be view by moderators only. These comment areas are attached to each idea so you can have private discussions with other moderators out of view of the author and other challenge users. This can be useful for holding conversations around moderation decisions on specific ideas.
You will also have access to a "Moderators and authors" area, which allows for a similar level of privacy, but also includes the author(s) of the current idea. Messages you leave here will generate notifications to the idea author(s) so they can respond.
Ensure that when posting a comment, the correct area is selected. The submit button will show appropriate text so you can be sure which area your comment is going to appear.