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.

Kernel
Advanced partition selection ( CONFIG_PARTITION_ADVANCED ) and EFI GUID Partition support ( CONFIG_EFI_PARTITION ) must be enabled:

Characteristics
will show it with the boot, esp flags:

For creation instructions see Handbook

will show it with partition code EF00:

Size considerations
100 MB should be enough for some kernels and boot manager and dual-booting Windows 10.

Mount point
Gentoo Handbook creates the ESP as Handbook:AMD64/Installation/Disks and mounts it at.

Standard layout
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.

UEFI boot items
Computers with UEFI usually provide a boot menu and a configuration tool for creating, sorting or deleting boot items. The content of the ESP is visible to these tools and creating a boot item is like choosing the medium from a given selection, then surfing through the ESP and selecting the item, e.g vmlinuz-4.9.16-gentoo.efi.

Alternatively, efibootmgr can be used for generating the UEFI boot items.

External resources

 * EFI System Partition on Arch wiki