Option ROM concept

Definition from wiki:
An Option ROM typically consists of firmware that is called by the system BIOS.

Give an example of option ROM. BIOS want to initialize video card, but BIOS vendors don't have the data sheet of the video card. They have to call some functions in the Option ROM of video card, and initialize the VGA card.
Usually, functions in Option ROM need to be executed during the real mode.
Like coreboot, most of code are executing at protected mode. So it need to switch back to real mode in order to initialize the VGA.