The Sanskrit word Tapas means heat, or the generation of heat. It is used most often for the purifying effect of a yogi at strong spiritual exertion.
Within the International Nath Order that heat is worked as sadhana, not as a theatre of pain. The dhuni is a fire that produces another heat, and vibhuti is its ash. Shambhala Tapovan takes the same root: a grove of austerity. Tapas is the heat. Sadhana is the daily work. They are not one word. This is not a grade and not a list of ordeals.