Thank you Paul for your answer.
"FF" I supposed to be "frames", for a video of 25fps it should be from 00 to 24.
But the format could also be "HH:MM:SS,MMM", "00:10:15,100" where "SS.MMM" is "seconds and milliseconds".

The word document is shown in realtime while I'm typing for deaf people (but not only). The event/conference is recorded (audio or video, or both), so once it is finished I can instantly share the recorded media plus the subtitle file of the event.
So the TimeStamp format depends of the subtitle file format (.srt or similar).
I've not yet decided it because I'm still in a planning phase, and I don't know if Word Macros has some limitation to obtein that.