  We identify the exit path $\infty$-category of the reductive Borel-Serre
compactification as the nerve of a $1$-category defined purely in terms of
rational parabolic subgroups and their unipotent radicals. As an immediate
consequence, we identify the fundamental group of the reductive Borel-Serre
compactification, recovering a result of Ji-Murty-Saper-Scherk, and we obtain a
combinatorial incarnation of constructible complexes of sheaves on the
reductive Borel-Serre compactification as elements in a derived functor
category.
