The boot order list is read from a globally defined NVRAM variable. The boot sequence for UEFI consists of the following: Another example would be booting to an OEM-defined diagnostic environment if a critical error was discovered in the boot process. One example of a value added feature would be not loading a UEFI driver if booting failed the first time the driver was loaded. The platform firmware may also implement value added features in the boot manager if an exceptional condition is discovered in the firmware boot process. The platform firmware may add extra boot options or remove invalid boot options from the boot order list. The platform firmware must use the boot order specified in the global NVRAM variables for normal boot. The boot manager will attempt to load UEFI drivers and UEFI applications (including UEFI OS boot loaders) in an order defined by the global NVRAM variables. The UEFI boot manager is a firmware policy engine that can be configured by modifying architecturally defined global NVRAM variables.

Uefi boot manager