Some paper by Rezk?
arXiv:1102.0186 n-Relative Categories from arXiv Front: math.CT by C. Barwick, D. M. Kan We introduce, for every positive integer n, the notion of an n-relative category and show that the category of the small n-relative categories is a model for the homotopy theory of n-fold homotopy theories, i.e. homotopy theories of … of homotopy theories.
nLab page on Homotopy theory of homotopy theories