subtitles can be anything you want if you have an editor with subtitle functionality. it could be the dialogue from the video; it could be "thought bubbles" for

subtitles can be anything you want if you have an editor with subtitle functionality. it could be the dialogue from the video; it could be "thought bubbles" for the characters, or just ascii strings of whatever code. in a "good enough" sense end user subtitled video has potential use for steganographic methods.