You may view a review as unacceptable when it is overly thin and uninformative.  Senior PC members and Area Chairs are monitoring and also rating the reviews. In case reviews are overly thin and uninformative (as judged by the SPCs and ACs), they will either be expanded, or not be taken into account. Other reviews may be added in such cases. You may view a review as unacceptable because it is unethical. These are the two cases where you can use the confidential box in the rebuttal.