This image touches on the eastern margin of the dark region informally known as Cthulhu Regio, and passed at the bottom of the image over the mysterious, possibly cryovolcanic edifice Wright Mons, before reaching the terminator or day-night line.
It shows the relatively smooth, bright volatile ices from Sputnik Planum (at the edge of the mosaic) and the dark edge or “shore” of Cthulhu.
See the image of Pluto as reference. The mosaic image is taken roughly from the area within the red lines.
NASA/JHUAPL/SwRI