This a feature of the chosen USB controller that requires motherboard support.
To be brief, it allows the computer to act as either USB host OR as a USB peripheral. This allows for building your own USB devices for various purposes (webcam, external monitor, handmade handheld PC that can also be a controller for your big rig, and lots more if you get creative).
It is being pure Hell trying to find anything that supports it other than SoC devices. That’d be fine, ordinarily, but this computer will be part of a development kit for a VR app I am working on. Thus, it needs to support the common denominator for VR/game development: x86_64 with a dedicated GPU.
If the none of the Framework motherboards have this feature, please consider adding it in future revisions. There are a few controllers that support this feature, a big one being Synopsys DesignWare Core 3. One other recommendation I would make is to have a dedicated USB-C port for this. When the controller is in “peripheral/client” mode, it cannot be used to drive the bus, and anything attached to that port that is not another PC acting as USB host will not be detected.