The MacPro6,1 and iMac15/17 perform special functions on their specific OEM GPUs/Displays. The MacPro6,1 unloads 1 GPU (with no ports) and leaves one functional for it's pseudo CrossFire function. The iMac15/17s apply a special EDID for their 5K displays.
The following snippet is taken from /System/Library/Extensions/AppleGraphicsControl.kext/Contents/PlugIns/AppleGraphicsDevicePolicy.kext/Contents/Info.plist:
...
<string>com.apple.driver.AppleGraphicsDevicePolicy</string>...
Black Screen with MacPro 6,1 or iMac 15 or iMac 17 System Definition
The following snippet is taken from /System/Library/Extensions/AppleGraphicsControl.kext/Contents/PlugIns/AppleGraphicsDevicePolicy.kext/Contents/Info.plist:
Code (Text):
...
<string>com.apple.driver.AppleGraphicsDevicePolicy</string>...