From Avisynth wiki
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.
An example Stack16 image: