Right usage of TTML for video

Hello people.
I'm very new to TTML and FCC regularion brought me here. after short
surveying, still very confused about how to deal with TTML in modern
browser implementation.

I can see several links which seem to demonstrate TTML.
http://www.cwmwenallt.com/ttml/ttml-demo.htm
http://www.html5labs.com/HTML5CaptionDemo/

and they are all something like using track element.

<video width="640" height="480" controls>
  <source src="video.mp4" type="video/mp4" />
  <track src="ttml.xml" kind="subtitles"srclang="en" label="English" />
</video>

but I am not sure if that is right implementation based on current
standard. if it is, it's wired that track does not have mime type. and if
it's not, I don't see any HTML element who requires TTML as text subtitles.

so my question is where web developers are suppose to put TTML and on which
element should browser work to make it happen?

on second thought, javascript can request and parse specific xml(TTML here)
and display them being styled somewhere on the web page. can I say TTML is
only for this?

Any comments would be very helpful.

Thank you in advance.
Eunice