If you scroll down just a bit, the article provides a live demo so you can play with the system and see what’s happening. The “line numbers” (not a great name)

If you scroll down just a bit, the article provides a live demo so you can play with the system and see what’s happening. The “line numbers” (not a great name) are the column edges. So 1/3 goes from edge 1 (the left-most) to edge 3, spanning 2 full columns.