Subtitles are just text data with a time code corresponding to a point in the video, so the feature of "subtitles" can be used to store whatever text you want,

Subtitles are just text data with a time code corresponding to a point in the video, so the feature of "subtitles" can be used to store whatever text you want, including notes, even though keeping notes was not the intent of the feature. There is nothing requiring the text data to be either an exact transcription or translation of the audio data. In fact, some fansub groups have released "joke" or "troll" translations for fun on various anime videos. Once you generate your subtitle text file, you can easily merge it into the video file. There are a few different subtitle text formats but the only one I've really worked with is SSA/ASS. The MKV video container format allows you to store multiple "streams" (files) of ASCII or UTF-8 text synced by time codes to the video. The reason for multiple subtitle streams is so one video (container) file can have subtitles in multiple languages. This means if there's some hard limit (I'm not sure if there is) on the amount of text you can have at a single time code, you can get around the problem by adding another stream/file.