 This is the distance from the joint root to the triangle hypotenues of the triangle slide formed by putting a line the "RightOfProjection" point and the right toe undercut point.
According to AWS A.3 Standard Terms and Definitions, the theoretical throat is "the distance from the beginning of the joint root perpendicular to the hypotenuse of the largest right triangle that can be inscribed within the cross section of a fillet weld. This dimension is based on the assumption that the root opening is equal to zero." (AWS Standard Welding Terms and Definitions) 
