QTFF, ISO 14496-12 and elst-media_time value

[aymv]

1. What is the priority standard for decoding mov files?
2. What value should be set in elst/media_time? Can we check it somehow?

• In QuickTime File Format Specification media_time is DT value:
  

  The edit list atom provides the initial DT value if it is nonempty (nonzero).

• In ISO IEC 14496-12 media_time is CT value.
  

  The Edit List Box provides the initial CT value if it is non‐empty (non‐zero).

It looks like the media_time value can be calculated and checked with ct + ctts (for ISO IEC 14496-12). But how to interpret the next note?

Edits are not restricted to fall on sample times. This means that when entering an edit, it can be necessary to (a) back up to a sync point, and pre‐roll from there and then (b) be careful about the duration of the first sample — it might have been truncated if the edit enters it during its normal duration. If this is audio, that frame might need to be decoded, and then the final slicing done. Likewise, the duration of the last sample in an edit might need slicing.

Does this mean that the media_time may not indicate a specific PTS of the first frame but in the "middle" of two frames for example? But I did not find in the standard how to work with this situation and which of these two frames must be displayed in such a situation as the first frame.