Used to map well-known ski tours for nordic ascent, usually with skins. Implies piste:grooming=backcountry.
Descent is not necessarily mapped. Generally, descent happens near the ascent route for safety and terrain judgement. To map a descent, use piste:type=downhill with piste:grooming=backcountry.
Difficulty of a skitour can be mapped according to this scale. (Note that the scale does not grad the ascent, but the descent.)
While skitours are mapped, some mappers and skitourers are doubtful of the usefulness. Itineraries vary a lot, which is part of the fun in skitouring. General opinion is that you are better served with a topo.