The Governing Vessel is not one of the twelve principal channels. It belongs to the eight extraordinary vessels, a separate class the tradition describes as reservoirs rather than as circuits: they do not pair with an organ, they do not take a place in the horary clock, and the classical image for them is of ditches that take the overflow when the twelve rivers run high.
Two of the eight are exceptions to their own class, and the Governing Vessel is one. Along with the Conception Vessel it carries its own points rather than borrowing points from the twelve, which is why it is usually listed alongside them and why the atlas holds it here. It runs up the midline of the back from the coccyx, over the crown, to the upper lip, and the tradition calls it the sea of yang, the vessel that governs all the yang channels.
Its second life is in Daoist internal alchemy, neidan, where the Governing Vessel and the Conception Vessel together form the circuit known as the microcosmic orbit: breath and attention are described as being circulated up the back and down the front in a closed loop. That practice is a meditative discipline within a living religious tradition, described here rather than instructed, and the atlas records it as practice rather than as technique to be attempted from a reference entry.