  The holographic entropy bound is discussed in cosmology. We try to define a
special class of spatial systems in cosmology, inside which the entropy of
matter satisfies the holographic entropy bound throughout the evolution of the
universe. We first take a universe composed of black holes as an example and
demonstrate that such a special class of systems does exist. Then, from the
perspective of entropy inside the co-moving volume, we find that there exists a
universal definition for this special class of spatial systems, i.e., the
definition is independent of the components of the universe. Finally, we prove
that in some specific cosmological models, multiple definitions can arise for
this special class of spatial systems, and we analyze the underlying reasons
for the emergence of these multiple definitions.
