The concept of booting the bootloaders on UEFI is straight and simple as putting the `.efi` bootloaders on the EFI system partition. The `.efi` file then load the corresponding operating systems. However, I still can't get why MBR needs an active partition to boot the OS.

From my study so far, the MBR bootstrap code resides on the first 446 bytes of the disk, which will be automatically executed by the BIOS after POST (as a de-factor standard). It can load the OS directly, but most of the time, the code is so small that its main job is to load the actual bootloader lying somewhere on another partition.

Why do we need to set a partition as active to boot from it? Theoretically, the MBR bootstrap code can run whatever, from wherever it feels like.