  Recently, a layered ferroelectric CuInP2Se6 was shown to exhibit domain walls
with locally enhanced piezoresponse - a striking departure from the
observations of nominally zero piezoresponse in most ferroelectrics. Although
it was proposed that such "bright" domain walls are phase-boundaries between
ferri- and antiferroelectrically ordered regions of the materials, the physical
mechanisms behind the existence and response of these boundaries remain to be
understood. Here, using Landau-Ginzburg-Devonshire phenomenology combined with
four sub-lattices model, we describe quantitatively the bright-contrast and
dark-contrast domain boundaries between the antiferroelectric, ferroelectric or
ferrielectric long-range ordered phases in a layered
ferroelectric-antiferroelectric ferroics, such as CuInP2(S1-ySey)6
