Hi Pilou,
the nod file sweeps either a round or a rectangular pipe on the paths (rails) you create.
You can select numerous paths and cycle one by one thro them with the knob.
When your happy with the style of pipe shape (round or rectangular) you want press apply.
With the knob select the next path and so on.
Perhaps the gif below will help it as 2 paths.
Cheers
Barry