Letters by Disk Signature
Every drive which has a partition table has a 32 bit long signature. If zero then Windows asks if the disk shall be "initialized" when opening the Disk Management.
It's a random value which is used for identifying drives, e.g. it is part of the UniqueID used by the Windows Volume Manager.
Since V4.7.1 USBDLM can use the signature as criteria. It might be helpful if drives have no other differences.
The value is interpreted as little endian ULONG value, so if you find on the disk offset 1B8h the bytes F5 17 24 4D then the hex value is 4D2417F5.
UsbDriveInfo shows the signature like this along with the other disk properties:
Disk Signature = 4D2417F5 (F5,17,24,4D)
Sample for having this drive on Z:
[DriveLetters]
DiskSignature=4D2417F5
Letter=Z
The signature must be noted with eight digits or with wildcards.
On GPT drives the disk signature is usually null, here the Disk ID Guid can be used instead, like so:
[DriveLetters]
GptDiskIdGuid={ac6ce627-1b3a-4b9d-9383-94842785585d}
Letter=Z
UsbTreeView shows it in the disk's properties.