You always heal perfectly. What does this mean? If you have your arm chopped off, it will grow back. If you get sick with any ailment, you will get better. If you slice off someone else's arm and sew it to your empty arm socket, it will heal onto you, and soon be in perfect working order. If you slice off someone else's arm and sew it to your torso, it will also heal to working order, and you'll have a functional extra arm. If you use the limb or body part of an animal to do the same, it will also work. If you have your head sliced off and stuck to another persons body, you will also heal perfectly, or even if your brain is removed and placed in another skull.
As far as it is possible to tell, the only limit is that some part of your brain must be present (you aren't sure which, not wanting to risk excising any part of that organ for a test), and that the healing takes time, and that it only seems to work on things not immediately fatal. For example, if you are shot in heart, you will die unless quickly stabilised and attached to an artificial heart.