If the change in the orbital kinetic energies and orbital-wise partial charges of two orbitals from different molecular structures are below so-called DOS thresholds, these orbitals can be regarded as "unchanged" during the reaction and are assigned to the environment. All remaining orbitals are then assigned to the active system and can be described with a more accurate method, e.g., a correlated wavefunction method as in wavefunction-in-DFT embedding. This DOS methodology was combined with the multi-level DLPNO-CCSD(T) ansatz by Sparta et al. leading to the DOS-based multi-level DLPNO-CCSD(T 0 ) approach with a semi-canonical perturbative triples correction [(T 0 )]. Orbital pairs can be categorized into different levels depending on the sets to which the individual orbitals belong. For this purpose, sets of orbitals can be used either based on their population on manually selected atoms or automatically partitioned with consecutive DOS thresholds. In the simplest case of two sets of orbitals, three levels are created: orbital pairs for which both orbitals belong to the first set, both to the second set, and the case where one orbital belongs to the first and one to the second set (referred to as interfragment pairs in Ref. 17). Then, for each level, different DLPNO thresholds can be used which allows to focus the computational effort on the reaction center. In contrast to the manual selection, the automatic DOS scheme allows for targeting a definitive accuracy. However, within projection-based DLPNO-CCSD(T 0 )-in-DFT embedding and multi-level LCCSD(T 0 ):LMP2, large errors were found for the interaction between fragments using a DOS-based selection. This suggests that orbitals which are almost unchanged during a chemical process may still contribute significantly through their pair correlation with other orbitals. This problem has been overcome by the pair-selected multi-level DLPNO-CCSD(T 0 ) ansatz. Based on the DOS algorithm, bijective orbital-set maps are constructed using the DOS criteria for orbital kinetic energies and orbital-wise partial charges, where multiple increasingly tighter DOS thresholds are used to obtain small orbital set sizes. Since some orbitals do not fulfill these criteria even for the loosest selection threshold, a set of "unmappable" orbitals is defined.