"Default track"-flag meaning

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If you're talking about the actual flag on a media item, a track being set as default means it will be played as a first option unless you have a setting (such as play a language regardless of defaults). This is not always true with Jellyfin since I know using external players breaks this behavior and you'll generally see the first audio track played regardless of a) what you chose in the GUI and b) your language preference. MXPlayer seems to be better about respecting what you choose in the GUI.

Now that brings up another point: the track displayed in the Jellyfin web GUI should be the default track. Why Matroska lets you set multiple default tracks I don't know and I'm not sure what behavior Jellyfin exhibits in this case. Also, players don't have to respect any setting. They could just always choose Japanese and make you live with it, however, most do their best.