TrackId

Identifies one track for the life of one opened media item.

This is the stream index inside the container for tracks that came from the container, and a negative value for tracks the application added, for example an external subtitle file. A teletext stream lists one track per subtitle page (#510): its first page keeps the stream's index and each further page has an id of its own above every stream index. All of them are opaque to callers: pass back what Tracks gave you.