The UEFI-Bios:
The UEFI-Bios of your motherboard will look for, and use, the ESP’s it finds on your system to load boot-loaders for your operating system(s).
- The order of devices that are searched for that partition is dependent on the physical port your storage device connects to your motherboard.
This order can be modified in some BIOS’es but not all. - This special purpose partition (ESP) is meant to hold all bootloaders for your system and must be formatted as a FAT file system(UEFI-Specs §12.3 File System Format) (eg. fat32(vfat)) without encryption.
- It is important to note that the UEFI-Bios will only consider the first ESP per storage device.(UEFI-Specs §12.3.2 Partition Discovery)
- The entries and order of the boot-loaders is kept in NVRAM and can be accesed via the
efibootmgrcommand on most Linux distro’s these days, see man efibootmgr for it’s usage. - When Secure-Boot is enabled in the UEFI-Bios settings, these boot-loaders need to be cryptographically signed by at least one signature that is stored inside your NVRAM.
Otherwise you will get a red message at system start and the UEFI-Bios will refuse to start that boot-loader.
Feel free to comment with your opinions and eventual corrections. 
