This creates an hierarchy structure where each level couples to the levels above and below. In this way, non-Markovian feedback can occur between the system and bath via the ADOs. An abstract volume can be deduced from the hierarchy structure, which while in principle is infinite, becomes finite by termination of the ADOs based on Markovianity constraints, equation ( ). This constraint seals the hierarchy volume. The entire hierarchy structure defines the nature of the system bath interaction, but only one level of the hierarchy has physical meaning: the zeroth layer -the reduced density operator.