The number is frames. Each second is made up of a number of frames, this depends on the settings.

As I'm in the UK, my clips are 25 Frames Per Second (FPS).

thanks for the answer, but this is far away from my imagination. when it goes with hh.mm.ss, how can it turn to fps? why fps is there near the duration instead of miliseconds which i need most. beside that there are lots of peaople from differet regions and millions of videos with 23,97, 24, 25, 60 fps. fps is another question. what is the difference between the fps at settings and the customized/quality option? i can already set the fps at customized/quality option

Quote why fps is there near the duration instead of miliseconds which i need most. beside that there are lots of peaople from differet regions and millions of videos with 23,97, 24, 25, 60 fps.

That's exactly why the time format is hh:mm:ss:ff

If you set your project frame rate to 24fps, the final digits will go from 0 to 23 before adding 1 to the seconds column. You can't arbitrarily set a millisecond value because there are only 24 frames per second. Each one lasts about 4.2mS - so there's no way to ever reach a location exactly 15 or 40 or 98mS into the clip.

If you set the project frame rate to 60, each frame lasts 1.67mS. That will give you more precise control, but you still can't reach every millisecond.

In general, it's better to produce your video using the same frame rate as your source clip to avoid the potential issues caused when PD has to drop or insert frames as the clip moves along. Setting a different frame rate doesn't affect the quality of the individual produced frames, but there can be noticeable glitches during playback if some obvious motion is occurring right where a frame is added/removed.

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