The Internet Standards Process (RFC 2026) describes the following role for the IAB within the IETF Appeals process:
Should the complainant not be satisfied with the outcome of an IESG review, an appeal may be lodged to the IAB. The IAB shall then review the situation and attempt to resolve it in a manner of its own choosing and report to the IETF on the outcome of its review.
If circumstances warrant, the IAB may direct that an IESG decision be annulled, and the situation shall then be as it was before the IESG decision was taken. The IAB may also recommend an action to the IESG, or make such other recommendations as it deems fit. The IAB may not, however, pre-empt the role of the IESG by issuing a decision which only the IESG is empowered to make.
The IAB decision is final with respect to the question of whether or not the Internet standards procedures have been followed.