If you want to keep the subtitles as a togglable track without losing quality, you can embed them into the container.
If you need to edit the text or timing, you can extract the .ass stream using tools like or specialized extractors to save it as a standalone file. Integrity & Repair aass (3).mp4
ffmpeg -i "aass (3).mp4" -vf "subtitles=aass (3).mp4" output.mp4 . If you want to keep the subtitles as
A video file with integrated or external Advanced Substation Alpha (.ass) subtitles. aass (3).mp4
Below is a technical report on this file type and how to handle it based on common user scenarios. Filename: aass (3).mp4 Container Format: MPEG-4 Part 14 (.mp4)