We introduce a new geometrical invariant of CR manifolds of hypersurface type, which we dub the Levi core of the manifold. When the manifold is the boundary of a smooth bounded pseudoconvex domain, we show how the Levi core is related to two other important global invariants in several complex variables: the Diederich--Forn{ae}ss index and the DAngelo class (namely the set of DAngelo forms of the boundary). We also show that the Levi core is trivial whenever the domain is of finite-type in the sense of DAngelo, or the set of weakly pseudoconvex points is contained in a totally real submanifold, while it is nontrivial if the boundary contains a local maximum set. As corollaries to the theory developed here, we prove that for any smooth bounded pseudoconvex domain with trivial Levi core the Diederich--Forn{ae}ss index is one and the $overline{partial}$-Neumann problem is exactly regular (via a result of Kohn and its generalization by Harrington). Our work builds on and expands recent results of Liu and Adachi--Yum.
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