Safety-critical real-time standards define several criticality levels for the tasks (e.g., DO-178B - Software Considerations in Airborne Systems and Equipment Certification). Classical models do not take into account these levels. Vestal introduced a new multiple criticality model, to model more precisely existing real-time systems, and algorithms to schedule such systems. Such task model represents a potentially very significant advance in the modeling of safety-critical real-time systems. Baruah and Vestal continues this investigation, with a new algorithm under fixed and dynamic priority policies. In this paper, we provide some results about the optimality of Vestal's algorithm and analyze an interesting property of this algorithm. We also adapt sensitivity analysis developed by Bini et al. for multiple criticality systems.