Stack16

Stack16 is a workaround format commonly used to process 16-bit content in AviSynth's native 8-bit environment.

As explained in the Dither documentation:

As Avisynth doesn't support natively high bit depth, a specific format named 'stack16' is introduced. The picture is made of two parts: one containing the highest 8 bits (MSB) for each pixel, stacked on top of another containing the lowest 8 bits (LSB). When previewing the video stored in stack16 format, the top part looks quite like the original picture and the bottom one generally looks like psychedelic garbage.