The Transformation constraint lets you move/rotate/scale a constrained object/bone by moving/rotating/scaling the target object/bone kind of like the Copy Loc/Rot/Scale constraints except that you can make the constrained bone rotate by moving the target bone or scale by rotating or any combination you can think of set within the min/max values for an axis .
You can also re-map the X/Y/Z values to any of the three axes, so for example you can make a transformation (move) of the target bone N value in the X axis but make the destination/owner move the same N value in the Y or Z axes .