Kindly explain how Current thread realizes that user is Authenticated ( How User.Identity.IsAuthenticated gets true?)

Kindly explain how Current thread realizes that user is Authenticated ( How User.Identity.IsAuthenticated gets true?)
When is that actually IsAuthenticated gets value true (normally its read only) ? 
In FormsAuthentication - setting up a cookie (.AUTH) for the next request - sets IsAuthenticated = true?
Is it any different for ClaimsIdentity?

Thanks in advance..