: MP4 files do not support bitmap-based subtitles like PGS (from Blu-ray) or VobSub (from DVD) as "soft" (toggleable) tracks.
: The build volume is limited to 9.8″ L x 8.2″ W x 8.6″ H (250mm x 210mm x 220mm). Psub.mp4
If "Psub" refers to "Picture Subtitles" or "PGS Subtitles" in the context of an MP4 file, there are specific technical limitations for this container: : MP4 files do not support bitmap-based subtitles