I read reference of SysytemInfo.deviceUniqueIdentifier.
This reference wrote that concern about value SysytemInfo.deviceUniqueIdentifier return, on pre-iOS 7 and iOS 7.
So, I want to know about value SysytemInfo.deviceUniqueIdentifier return, on iOS 8+.
I’m not entirely sure what it returns on that platform, however I can advise you do not use it, Apple does not permit applications to access this devices unique identifier anymore for security/privacy reasons. It will throw up an error when you attempt to submit your application to the store.
I believe you can still however use the advertising identifier, however be aware that users can opt to reset/change this at any time.