Is this supposed to be shibari tan? The lines look too even to be from rope, more like something flat. The skin veins suggest the artist is sufficiently detail-oriented to want to get that right.

I mean, i can't think about other things that could create similiar tan. Also visible veins like that are mark of extreme pressure that was put for prolonged time on breasts, which works with the idea that it was shibari.
Also i don't think that uniform rope-pattern is possible in situation like that, she was probably squirming a lot and ropes were moving.