The classifying diagram was defined by Rezk and is a generalization of the
nerve of a category; in contrast to the nerve, the classifying diagram of two
categories is equivalent if and only if the categories are equivalent. In this
paper we prove that the classifying diagram of any category is characterized in
terms of classifying spaces of stabilizers of groups. We also prove explicit
decompositions of the classifying diagrams for the categories of finite ordered
sets, finite dimensional vector spaces, and finite sets in terms of classifying
spaces of groups