Exporting a Generic Image Sequence will output sequence of images, considering the selected 3D layout, aspect ratio and size, for each frame in the source video. And for the moments with no subtitles on the screen, the images will be fully transparent so they do not interfere in any way with the original picture of the material.
The Generic Image Sequence export will preserve the smooth segments transition segments in addition to its most important role: such an image sequence can be loaded and used with virtually every NLE system and guarantees the excellent quality of the 3D text.
Presumably, the number of exported images will be extremely large ~100k for 1 hour long movie and we suggest to switch the Export uncompressed images option off first, which will create a .ZIP archive instead.