  We examine the conditions in favor and necessity of a realistic multileveled
description of a decohering quantum system. Under these conditions approximate
techniques to simplify a multileveled system by its first two levels is
unreliable and a realistic multilevel description in the formulation of
decoherence is unavoidable. In this regard, our first crucial observation is
that, the validity of the two level approximation of a multileveled system is
not controlled purely by {\it sufficiently low temperatures}. We demonstrate
using three different environmental spectral models that the type of
system-environment coupling and the environmental spectrum have a dominant role
over the temperature. Particularly, zero temperature quantum fluctuations
induced by the Caldeira-Leggett type linear coordinate coupling can be
influential in a wide energy range in the systems allowed transitions. The
second crucial observation against the validity of the two level approximation
is that the decoherence times being among the system's short time scales are
found to be dominated not by the resonant but {\it non-resonant} processes. We
demonstrate this in three stages. Firstly, our zero temperature numerical
calculations reveal that, the calculated decoherence rates including
relaxation, dephasing and leakage phenomena show, a linear dependence on the
spectral area for all spectral models used, independent from the spectral shape
within a large environmental spectral range compared to the quantum system's
energies. Secondly, within the same range, the decoherence times only have a
marginal dependence on the translations of the entire frequency spectrum.
Finally, the same decoherence rates show strong dependence on the number of
coupled levels by the system-environment coupling.
