Explanation
The device must contain a system partition. On GPT drives, this is known as the EFI System Partition, or the ESP. This partition is usually stored on the primary hard drive. The device boots to this partition. The minimum size of this partition is 100 MB, and must be formatted using the FAT32 file format. This partition is managed by the operating system, and should not contain any other files, including Windows RE tools.

Note:

For Advanced Format 4K Native drives (4-KB-per-sector) drives, the minimum size is 260 MB, due to a limitation of the FAT32 file format. The minimum partition size of FAT32 drives is calculated as sector size (4KB) x 65527 = 256 MB. Advanced Format 512e drives are not affected by this limitation, because their emulated sector size is 512 bytes. 512 bytes x 65527 = 32 MB, which is less than the 100 MB minimum size for this partition.

Other utility partitions:

Any other utility partitions not managed by Windows must be located before the Windows, data, and recovery image partitions. This allows end users to perform actions such as resizing the Windows partition without affecting system utilities.

Protect end users from accidentally modifying utility partitions by identifying them using a GPT attribute. This prevents these partitions from appearing in File Explorer.

To set partitions as utility partitions

When you're deploying Windows by using DiskPart, use the attributes volume set GPT_ATTRIBUTE_PLATFORM_REQUIRED command after you create the partition to identify the partition as a utility partition. For more information, see the MSDN topic: PARTITION_INFORMATION_GPT structure.
To verify that system and utility partitions exist

Click Start, right-click This PC, and then click Manage. The Computer Management window opens.
Click Disk Management. The list of available drives and partitions appears.
In the list of drives and partitions, confirm that the system and utility partitions are present and are not assigned a drive letter.
Windows partition

The partition must have at least 20 gigabytes (GB) of drive space for 64-bit versions, or 16 GB for 32-bit versions.
The Windows partition must be formatted using the NTFS file format.
The Windows partition must have 16 GB of free space after the user has completed the Out Of Box Experience (OOBE) and Automatic Maintenance has completed.
Recovery tools partition

This partition must be at least 300 MB.

The Windows Recovery Environment (Windows RE) tools require additional free space:

A minimum of 52 MB is required but 250 MB is recommended, to accommodate future updates, especially with custom partition layouts.
When calculating free space, note:

The recovery image, winre.wim, is typically between 250-300MB, depending on what drivers, languages, and customizations you add.
The file system itself can take up additional space. For example, NTFS may reserve 5-15MB or more on a 750MB partition.
This partition must use the Type ID: DE94BBA4-06D1-4D40-A16A-BFD50179D6AC.

The recovery tools should be in a separate partition than the Windows partition to support automatic failover and to support booting partitions encrypted with Windows BitLocker Drive Encryption.

We recommend that you place this partition immediately after the Windows partition. This allows Windows to modify and recreate the partition later if future updates require a larger recovery image.

Data partitions

The recommended partition layout for Windows 10 does not include data partitions. However, if data partitions are required, they should be placed after the Windows RE partition. This allows future updates to Windows RE to grow the Windows RE partition by shrinking the Windows partition.

This layout makes it more difficult for end users to remove the data partition and merge the space with the Windows partition. To do so, the Windows RE partition must be moved to the end of the unused space reclaimed from the data partition, so that the Windows partition can be extended.

Windows 10 does not include functionality or utility to facilitate this process. However, manufacturers can develop and provide such a utility if PCs are shipped with data partitions.

Partition layout

The default partition layout for UEFI-based PCs is: a system partition, an MSR, a Windows partition, and a recovery tools partition.

This layout lets you use Windows BitLocker Drive Encryption through both Windows and through the Windows Recovery Environment.

Sample files: configure drive partitions by using Windows PE and DiskPart scripts

For image-based deployment, boot the PC to Windows PE, and then use the DiskPart tool to create the partition structures on your destination PCs.

Important

To avoid bare metal recovery boot issues due to partition size, manufacturers should allow the bare metal recovery feature's auto generation script to create the partition used for the recovery WIM. If manufacturer's want to use a custom DISKPART script to create partitions, the recommended minimum partition size is 990MB with a minimum of 250MB of free space.

Answer
In the image above, the extensions are .efi. From the explanation below, .efi is associated with GPT partitioning.