Vibhuti is the Sanskrit word for sacred ash associated with Shiva. It is produced in the fire of the dhuni. The common mark is the tilak of three horizontal lines. Lines, moons, and a full covering of the body are all found.
Within the International Nath Order the ash is work, not costume. Placement matters in two ways: the intention of the spot, and the Shakti in ash that was properly made. It is used to settle the mind. In initiation the Initiator’s touch is fixed by vibhuti on several parts of the body. The fire is the dhuni. The mark is the tilak. This page names the ash.