Sarvagni is the fourth dimension of fire. The other three dimensions are Jatharagni, Chitagni and Bhutagni.
Beyond this, there is something called sarvagni, because, according to modern science, the physical dimension of existence is less than five percent. That means if you know the entire physical dimension of the universe, you know only five percent of existence. Sarvagni touches the dimension where there are no elements, where there is no creation as you know it, or in other words, where there is no physical nature.
Generally, a Yogi who wants to access the nature of life will not interest himself in jatharagni, chitagni, and bhutagni. He will focus only on sarvagni, because this is the ultimate fire – but it is a cool fire. Jatharagni is a very obvious fire. Chitagni is less obvious but very much there. Bhutagni is not so visible at all but very much there. Sarvagni can hardly be felt, but without it, nothing would happen. It is the fundamental and ultimate fire that encompasses all other fires.