The positional tolerance x is defined as the distances between the actual terminal positions and the corresponding matrix points of which locations are determined by a terminal pitch e and the 2nd and 3rd datum AB derived from all of the terminal positions using the Least Squares Method. This is acceptable whenever the terminals are included in the circle whose diameter is the sum of maximum terminal diameter and tolerance x (φ0.05 mm).
See "Measuring method for Package Dimensions of BGA EIAJ ED-7304"