Our resident graphics expert is out for the next two weeks on holiday. I'll try to find someone who can answer you question in the mean time, but it might have to wait until Mike gets back in early January.
In our company, we have a device, that is composed of PC, a touch screen connected to HD15 (VGA) PC port, and external monitor connected to DisplayPort through a device to transfer the video signal across several meters.
We use CUISDK to create custom resolution (1920x480) and detect the primary monitor (touch screen) in the graphics plataform "HD Graphics 4600". We generated a project and worked properly, and we got uidMonitor and dwDeviceType.
We are currently certifing the new platform "HD Graphics 530", with the same project, we dont recover any data that makes sence.
As a workarround we have used operation system function like "EnumDisplayDevices" to detect the primary monitor (touch screen), this is working in both plaforms. When we did the tests the returned values where not the expected ones, as you can see here: