The tl;dr is it's a standard mini ITX-sized motherboard for the Raspberry Pi Compute Module 4. It has a Broadcom PCIe switch that it uses to power 4 mini PCIe s

The tl;dr is it's a standard mini ITX-sized motherboard for the Raspberry Pi Compute Module 4. It has a Broadcom PCIe switch that it uses to power 4 mini PCIe slots, 4 M.2 E-key slots, 1 M.2 M-key slot, a x16 slot in the standard ITX location, and an edge x1 slot. It also has dual/triple-redundant 12v power supplies (barrel, aux +/- ATX, and PoE++ header, though it doesn't use a 24-pin ATX connector, so you need to power it with an adapter or via wall wart (I used a 96W barrel plug adapter). Through the course of testing this thing, I also identified a bug in the PWM fan controller circuit—and apparently it's also a small bug present on the CM4 IO Board's implementation, but typically not a fatal one.