Participants with the Viewer role in a discussion (see: Participant Rights and Roles) are not able to comment on claims directly. Following the steps to comment on a claim instead suggests a comment, which can be accepted by any Admin. Until a suggested comment is accepted, it is only visible to the author and discussion Admins.
Once an Admin has accepted the suggested comment, it is visible to all users as a regular comment is. Admins can also respond to suggested comments, for example, if they feel it needs clarification, in which case the suggested comment can be edited and resubmitted.