Every other channel in Chinese medical theory, whether one of the twelve regular meridians or one of the remaining seven extraordinary vessels, runs vertically along the length of the body. The Girdle Vessel, Dai Mai, is the single exception, circling the waist horizontally like a belt, which is exactly what its name means. Classical descriptions place its origin just below the free end of the twelfth rib, from which it loops fully around the torso.
That geometry gives the Girdle Vessel a structural job none of the vertical channels can perform on their own: binding the long, vertical channels together at one horizontal band, so that a disturbance affecting the coordination between the upper and lower body is traditionally read as a sign the Girdle Vessel itself, rather than any single vertical channel, has become disordered.
The classical point associations follow from the geometry. Three points on the Gall Bladder channel sit on the Girdle Vessel's course around the waist, one of them named Daimai after the vessel itself, and the Gall Bladder channel is the only regular meridian with a substantial lateral, wrapping component of its own, which is why the two are so closely tied in classical description.
In the eight confluent point scheme used to open the extraordinary vessels in treatment, the Girdle Vessel is reached through Gall Bladder 41 on the foot and paired with Triple Burner 5 on the arm, so that a channel encircling the waist is worked from the extremities rather than from the band it describes.