According to page 194, there are some types of magic, mostly associated with magic items, that don't belong to one of the four main traditions of magic. Arcana is tied to Arcane spells, Nature for Primal, Occultism for Occult, and Religion for Divine, but what skill corresponds to "none of the above"?
As far as I'm aware, that's either up to the
GM, or any of those 4 skills. As in act 2, when trying to identify the magic puzzle circle thing, you could use any of those skills, I think it's like that.