x1 is such that its directed axis or primary 'face' is oriented/faces/aimed in the direction of/toward point x2 according to convention x3
x3 determines what is the axis that is being implicitly considered and what its direction is; for example, if x1 is an actually-oriented circular loop then a convention x3 which specifies that the line through its center which does not intersect the loop as the axis and the right-hand rule as providing the direction will determine x2. x2 is not a direction but any point in such a direction, so no frame of reference is necessary (the statement holds equally true in all frames of reference, even if the labels are different). See also: farka.