To visualize: Take a standard pipe. Instead of laying it flat (1G) or vertical (2G), you tilt it so it sits at a 45° angle. More critically, . The welder must move around the stationary pipe, welding in all quadrants: flat, horizontal, vertical, and overhead, all within a single joint.
Do not confuse with 6GR . The "R" stands for Restriction Ring . what is 6g in welding