The headers [main], [extra] and [side] mark the start of the main, extra and side deck respectively. The [unknown] header is for a field that I haven't figured out yet, seems to have no effect either.
In order to load an MKV file, it is necessary to create a new instance of the OpenK4APlayback class. Note that if the is_looping flag is set, the stream will not stop playing at the EOF of the stream.