Min_boxes (anchors) calculation

Hello!
In fd_config.py there is a following line: min_boxes = [[10, 16, 24], [32, 48], [64, 96], [128, 192, 256]]
As far as I understand, these are anchors, actually four of them, but why the dimensions differ (either 2 or 3 numbers)? And what do these numbers imply?