Question about VID_train_15frames.txt

Hi，
I have a question about your VID_train_15frames.txt file.
In each line, there are four parameters.
for example:
train/ILSVRC2015_VID_train_0000/ILSVRC2015_train_00000000 1 10 300
Apparently the first one is the index of video.
However, I am wondering what the three left means and how they works.
In your codes, you call them frame_id,frame_seg_id,frame_seg_len.
