Analogues of centralizer subalgebras for fiat 2-categories and their 2-representations

The main result of this paper establishes a bijection between the set of equivalence classes of simple transitive $2$-representations with a fixed apex $mathcal{J}$ of a fiat $2$-category $cC$ and the set of equivalence classes of faithful simple transitive $2$-representations of the fiat $2$-subquotient of $cC$ associated with a diagonal $mathcal{H}$-cell in $mathcal{J}$. As an application, we classify simple transitive $2$-representations of various categories of Soergel bimodules, in particular, completing the classification in types $B_3$ and $B_4$.