A unique device identifier. It is guaranteed to be unique for every device (Read Only).
iOS: uses UIDevice.identifierForVendor (https://developer.apple.com/documentation/uikit/uidevice/1620059-identifierforvendor).
Android: SystemInfo.deviceUniqueIdentifier always returns the md5 of ANDROID_ID. (See https://developer.android.com/reference/android/provider/Settings.Secure.html#ANDROID_ID). Note that since Android 8.0 (API level 26) ANDROID_ID depends on the app signing key. That means "unsigned" builds (which are by default signed with a debug keystore) will have a different value than signed builds (which are signed with a key provided in the player settings). Also when allowing Google Play to sign your app, this value will be different when testing locally built app which is signed with the upload key and app downloaded from the Google Play which will be signed with the "final" key.
Windows Store Apps: uses AdvertisingManager::AdvertisingId for returning unique device identifier, if option in 'PC Settings -> Privacy -> Let apps use my advertising ID for experiences across apps (turning this off will reset your ID)' is disabled, Unity will fallback to HardwareIdentification::GetPackageSpecificToken().Id.
Windows Standalone: returns a hash from the concatenation of strings taken from Computer System Hardware Classes (https://msdn.microsoft.com/en-us/library/windows/desktop/aa389273(v=vs.85).aspx):
Win32_BaseBoard::SerialNumber
Win32_BIOS::SerialNumber
Win32_Processor::UniqueId
Win32_DiskDrive::SerialNumber
Win32_OperatingSystem::SerialNumber
Will return SystemInfo.unsupportedIdentifier on platforms which don't support this property.