Third, a wide range of extensions of sDFT/FDE is reviewed in Section 6. Major developments in recent years have addressed the replacement of NAKE approximations by projection techniques. An account of projection-based embedding (PbE), which can be regarded as a variant of density-based embedding with mutually orthogonal subsystem orbitals, in combination with correlated-wavefunction descriptions of the active system has been given by Manby, Miller and co-workers . Applications of PbE for transition metal complexes have been reviewed in Ref. . Here, we will discuss PbE techniques and their relation to sDFT in Section 6.1.