This term is a classic example of a legal translation problem. It was used in a medical context, where such a specific legal term was complete overkill, and the medical translator had trouble researching it, unsurprisingly.
The general context was: This patient had an injury, and the haftungsausfüllende Kausalität between the accident and the injury to health has been proved.
Before I go into detail, here is the short answer: haftungsausfüllende Kausalität (literally liability-fulfilling causation) means that the accident caused the injury. Thus the writer is actually explaining the technical term in context. A translator could write ‘It has been proved that the accident caused the injury’.
But in another context it might be necessary to explain the term. One port of call is my ancient edition of Hans Brox, Besonderes Schuldrecht – just occasionally, German students’ textbooks are the best resource – which has index entries:
Kausalität bei unerlaubter Handlung
Haftungsbegründend: if the defendant’s conduct caused the legal wrong (Rechtsgutverletzung) and the defendant is responsible for this.
Haftungsausfüllend: if the legal wrong caused the damage/loss/injury.
This is the German way of determining whether a defendant is liable for an injury. The common law looks at causation in a similar way, differing mainly in detail. The German approach is very theoretical, but so is the American approach.
Now I wonder what Markesinis and Unberath, The German Law of Torts, have to say about this.
French lawyers, paraphrasing Voltaire’s dictum about the existence of God, have often teased the Germans by saying that if causation did not exist as a subject it would have to be invented so that German lawyers would have something to exercise their minds.
Markesinis says that the Germans, like the Americans and the English but more clearly so than the French, take a bifurcated approach to problems of causation.
The first stage, haftungsbegründend above, requires that the defendant’s conduct is a conditio sine qua non of the hurt, which is better know to Common lawyers as the ‘but-for’ test.
By the way, when I write of the defendant, German texts refer to der Täter or der Schädiger.
There doesn’t seem to be a close equivalent to the second stage in English tort law. The question of foreseeability and directness of consequence arises. There is a term used in the USA in this context, and that is proximate cause. To quote another textbook, Kenneth S. Abraham, The Forms and Functions of Tort Law, ‘Use of the term “proximate cause” is unfortunate and misleading.’
Indeed, the term proximate cause has made it to Language Log, where it was discussed by Roger Shuy, my second-favourite writer there. Interesting links there too. The term came up when a lawyer was a juror and other jurors asked him to define the term. Tiersma is quoted as giving proximate and cause as problematic words.
I note that there is a discussion of this on ProZ. The discussions there are often helpful. Beate Luetzebaeck is very reliable (she’s trained as a lawyer in both common law and German systems). The asker chose proximate cause, which appears wrong to me, especially when contrasted with contributory cause – that’s not the distinction made in German.