  We study the ordered phases and the phase transitions in the stacked
triangular antiferromagnetic Ising (STAFI) model with strong interplane
coupling modeling CsCoCl$_3$ and CsCoBr$_3$. We find that there exists an
intermediate phase which consists of a single phase of so-called partial
disordered (PD) type, and confirm the stability of this phase. The low
temperature phase of this model is so-called two-sublattice ferri magnetic
phase. The phase transition between the PD phase and two-sublattice ferri
magnetic phase is of the first order. This sequence of the phases is
homomorphic as that in the three-dimensional generalized six-state clock model
which have the same symmetry of the STAFI model. By studying distributions of
domain walls in one dimensional chains connecting layered triangular lattices,
we clarify the nature of the phase transition and give an interpretation of
little anomaly of the specific heat.
