Video transparency — packed matte None — regular video Packed matte — top/bottom (color on top, matte below) Packed matte — side by side (color left, matte right)

For videos already exported with the color image and a black-&-white alpha matte packed into one frame (top/bottom recommended). Regular videos: leave as "None".

Or a separate mask image (static cutout)

Use one of the two. A mask is a plain black & white image the same size as your target — white where the video should show, black where the target should show. No special video export needed. You can also add or change this later in the editor.