The idea is simple. For FAA logging purposes, instrument time may be logged when the airplane is flown solely by reference to instruments. In the FAA universe, this means there is no outside visual reference available to the pilot. This is either because a view-limiting device is being used in the case of "simulated instrument time" or because meteorological conditions of limited visibility prevail during "actual instrument time". Black of night doesn't count.
As a practical matter, all jet flying is instrument flying if you're doing it right! Even when there's something useful to see out the windshield, the airplane is still controlled by reference to instruments AND whatever useful information can be sensed by looking out the window. How you do it and how you log it are not the same thing.
But if the question is what may be logged as "instrument time" under FAA rules, please see paragraph 1.