The system is very flexible and can be customized to fit your needs. Perhaps you only want the left hand to be IK and the right hand to use its FK position, but IK rotation. Feet are sometimes handled this way, sometimes not. When stepping on a very precise prop, you would want to turn on IK; when you are just running around, you would want to use the FK, so you do not end up with a bow-legged character (or the opposite).