[HELP] How to set a subtitle as the default track

[Selur]

What I meant was in the bonobono image(the source) the subtitle track is set to default meaning the subtitle will be played automatically with the video and audio when the player is started.

For the "default track" flag: The special consideration is that mkvmerge will automatically set this flag to "yes" for exactly one track of each track type (audio, video, subtitles). You can only prevent this by explicitly setting the "default track" flag to "no" manually for all tracks of a kind (e.g. for all subtitle tracks).
The "default track" flag tells the player that this track should be played unless the user overrides that decision somehow. You usually mark the original audio track with "default track" and leave it off for the rest of them, e.g. for the director's commentary or some translations you don't want [(e.g. The Lord of the Rings, mark "English" as "default track" but not "German" and "Director's comments (English)].
As a lot of users don't want subtitles shown by default, they tell mkvmerge not to set the "default track" flag for any subtitle track.
Now on to the "forced display" flag, in short "forced". If this is set to "on", this track must be played/shown no matter what the user selected for his preferences or what the player would normally chose to show/play. This is used seldom, e.g. only for a subtitle track that only contains the English translation whenever Legolas is talking Elbish.
"Forced" has nothing to do with "default track". If "forced" is set, the player must play that track no matter what "default track" is set to. In fact, normally a track that has "forced" set does not have "default track" set, though it is neither invalid nor undefined behavior.

source: https://gitlab.com/mbunkus/mkvtoolnix/wi...in-the-GUI