It is used to express a continued or ongoing action that started in past and continued until sometime in past. (Remember, an ongoing action in past which continued till some time in past) There will be a time reference, such as “since 1980, for three hours etc” from which the action had started. A sense of time reference is found in these sentences which shows that action had started in past and continued till some time in past. Such time reference or sense of reference is the identity of Present perfect continuous tense because it tells that action has started from a particular time in past or for some time period. For example, “He had been studying in this school since 2005”, so the it means that he had started his education in this school in 2005 and he studied in this school till sometime in past.