I would propose it is your digestive capability, your processing capability, your ability to integrate. Thus, I propose four grades of trauma:
1) Life trauma: this you can integrate into an existing sense of self with little modification. This would include the death at a normal age of a parent.
2) Life trauma that forces an improvement in your sense of self. Example would be Marine Boot Camp. Most people leave feeling a part of something larger than themselves, more energetic, and more disciplined. War, similarly, has this effect on many people; others it breaks–see number 4.
3) Life trauma that forces a degradation in your sense of self. An example would be a bad marriage, in which to keep things together you have to sacrifice parts of yourself that are important.
4) Life trauma you can’t integrate at all, like premature sexual experience/molestation, or “combat fatigue”. This goes into a hidden, timeless place which you can’t remember well, but which frequently sends out signals in all sorts of weird ways that it is still there and unprocessed.