The Refugee Review Tribunal, the Migration Review Tribunal and the AAT deal with merits review. Since 1 July 2015, the former two tribunals have merged into AAT, which is now the only merits review body.
In a merit review, the tribunal puts itself in the shoes of the original decision maker and review the merits of the decision by reconsidering the facts, law and policy aspects relevant to the original decision, and come to a decision as to whether the original decision is correct and therefore be affirmed, or whether a correct and preferable decision should be made. The Tribunal will consider the evidence from a fresh perspective.
Merits review is more or less your “second attempt” at the application.