There is the legacy BIOS (no UEFI at all), needs to boot from MBR disk. Data disk(s) can be GPT. Next, the Gen1 EFI, that proprietary type is almost extinct. Third, Gen2 UEFI, now common in PCs. It includes a CSM and legacy BIOS INT10 emulation. Can boot Windows 7 in EFI mode, like Gen1. Next round of UEFI, Gen3 (mostly found in newer Notebooks and Laptops) lacks the CSM and BIOS INT10 emulation, completely. It drops the Windows 7 compatibility. That's the type of UEFI discussed about, here.