We identify the exit path infinity-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 immediate consequences, 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.