Asus UEFI tells: "the VGA card is not supported by UEFI driver" and enables CSM (Compatibility Support Module: Legacy BIOS boot mode).
Enabling CSM doesn't make much sense as it won't help if you are using GPT or the OS requires UEFI to boot.
Background: I inserted an old ATI RV710 (HD 4x50) into my PC to build a multi-seat environment.
What it means is that the UEFI firmware detected a old video card without UEFI GOP code, so the CSM has been enabled to allow the legacy video BIOS code to be used. You can still boot in UEFI mode because UEFI GOP includes support for calling legacy video BIOS code through the CSM.
This just means that the UEFI mode BIOS does not fully support your video card, so it needs to enable CSM mode which can make it work using old BIOS mode controls. It's unfortunate, but not much you can do about it, just live with it.
You can still boot in UEFI mode, even though CSM compatibility is enabled.
Once the system is up this CSM mode does not really affect anything.
Cause: Secure Boot does not work with the driver used because it's not signed. Not sure why. As I did not install any additional ATI driver.
Solution: Disable Secure Boot. On my Asus Board I had to choose between "Windows" and "Other OS". Where Windows means Secure Boot is enabled, other - disabled. Great Naming.
You also should disable CSM. I don't understand why the UEFI automatically enabled CSM. As this does not help with the boot process if your system wants to boot via UEFI.