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. 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.