What does the 3rd labels of point mean in instance segments?

[yichaoshen-MS]

I found there is a 3-dims labels per point, first is the class-id, the second is instance-id, but I don't know the 3rd dim means. I guess it may be able to be named as "segment-id". because it usually generates items like "point2segment","segment2label". So, what does the 3rd dim in point's labels mean, or what does the segment represent?