Having a third participant that you can sue or subpoena is the important part. Is there a good word for this sort of thing? Facilitated by a third party with si

Having a third participant that you can sue or subpoena is the important part. Is there a good word for this sort of thing? Facilitated by a third party with significant assets subject to the legal system? I heavily favor using such parties for my personal financial affairs - it's not that I trust them more, but it's that I have more recourse when things go wrong.