EFI System Partition

The EFI system partition (ESP) is a fat formatted partition containing the boot loaders or kernel images for all installed operating systems.

will show it with the boot, esp flags:

For creation instructions see Handbook

will show it with partition code EF00:

There is a standard layout for the ESP. Vendors and distributions are supposed to put their stuff into vendor specific directories.

Here the Microsoft subtree - and also the Boot subtree - was created by an earlier installation of Windows 10 Creators Update. The Boot subtree is the fallback directory. If UEFI can't find any vendor specific directories it will boot from here. In a multiboot environment with properly set up vendor specific subtrees the Boot subtree can be deleted.

External resources

 * EFI System Partition on Arch wiki