A unique device identifier. It is guaranteed to be unique for every device (Read Only).
iOS: on pre-iOS7 devices it will return hash of MAC address. On iOS7 devices it will be UIDevice identifierForVendor or, if that fails for any reason, ASIdentifierManager advertisingIdentifier.
Android: SystemInfo.deviceUniqueIdentifier always returns the md5 of ANDROID_ID. (See https://developer.android.com/reference/android/provider/Settings.Secure.html#ANDROID_ID).
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.