Time expressions when ttp:timeBase="media"

Hello everyone,

I would like to get some clarification on how time expressions work when using ttp:timeBase="media".

The spec says:
If the time base is designated as media, then a time expression denotes a coordinate in some media object's time line, where the media object may be an external media object with which the content of a document instance is to be synchronized, or it may be the content of a document instance itself in a case where the timed text content is intended to establish an independent time line.

It is not clear to me whether a time expression denoting "a coordinate in the media object's time line" can imply something that is different from "time elapsed since the start of the media object, as counted by an external clock running in real time" or something to that effect.

Assuming ttp:timeBase="media" is specified, I would like to know:

1) Does a time expression always refer to elapsed time from the start of the media, as measured by an external clock running at real time?

If the answer to 1 is "always" then there is no problem -- TTML time expressions are always in sync with an external clock, even if the media object has some other kind of coordinate system which is not, such as a NTSC non-drop frame time code track.  (In NTSC non-drop frame, video time codes do not count at the same speed as an external clock would; they drift apart at 3.6 seconds per hour.)

If the answer to 1 is "not always" then I would like some further clarification:

2a) What exactly does "a coordinate in some media object's time line" mean?

2b) By what methods can such a coordinate be determined?

2c) If there are multiple possibilities for interpreting a coordinate, how does the TTML parser know which method should be used to interpret the time expression?

Thank you.