They're owned by the AAAs (All of which have an equal share), and is the basis of power for the Corporate Court. They, literally, control the Nuyen. Period. Fragging. Dot.
It's like a subsection of the Corporate Court, but cannot dictate to the AAAs in any way without the Court's order. Usually that would be to not extend credit for, say, Renraku needing Cred to pay the UCAS for liberating the Archology.
They're also the final voice in what is a Nuyen, what isn't. The buck, literally, stops there. If the Z-OG says your Nuyen is counterfeit, it's fake and you're going to meet Bubba The Love Troll.