Who uses a PCI-U Socket and for what?

Anybody worked with Supermicro’s PCI-U sockets.
PCI-U is described below and is about all I can found about this socket.

The board has two PCI-Express 2.0 x8 slots, one PCI-U (a so-called “universal slot” invented by Super Micro that lets a few different types of PCI-Express cards plug into it), one PCI-Express x4 slot, and three legacy PCI-X slots. This board also has dual Gigabit Ethernet ports, six SATA drive ports, and two Type A flash ports.

I plugged a USB PCI-e card into the PCI-U socket and it burned up the USB card power supply!
This socket is supposed to run PCI-E cards too so what is up with this socket and how and what can it really be used for.

It looks to me like an outdated malformed plan to add to the PCI-E functionality.
It seems to be unusable.