They are used in rods and bone hierarchies. These constrain the position of two particles relative to each other, displacing them along the X,Y,Z axis of a reference frame defined by an orientation quaternion. Stretch is minimized along the Z axis, while shear is minimized along the X and Y axis:
Amount of stretching allowed along the reference frame's Z axis.
Amount of shearing allowed along the reference frame's X axis.
Amount of shearing allowed along the reference frame's Y axis.