Действие, происходившее до определённого момента в прошлом. Рассматривается, как процесc. Действие в Past Perfect Continuous ПРЕДШЕСТВУЕТ действию в Past Simple:

He had been working for over an hour before his workmate turned up.
The few survivors looked painfully thin. They had been living on meager rations since the accident.
We had been driving for about an hour when the engine suddenly stopped.
The lake was near to bursting point as it had been raining heavily for weeks.
They had been sailing for seven days when they ran out of food and water.

Обычно, группа Continuous не употребляется со статичными глаголами (описывающими НЕ действие, А состояние):

Cтатичные глаголы:
1. ментальные
– мысли и мнения (agree, believe, doubt, guess, imagine, know, mean, recognize, remember, suspect, think, understand)
– чувства и эмоции (dislike, hate, like, love, prefer, want, wish)
– восприятие (appear, be, feel, hear, look, see, seem, smell, taste)
2. владеть (belong, have, possess, own)
3. измерять (measure, weigh)

I had known the secret for a long time by then (NOT I had been knowing the secret…)
My parents had had this car for about 10 years before the accident (NOT My parents had been having…)
We had never been happy together, I’m afraid. Therefore, divorce was completely natural. (NOT We had never been being happy …)

Past Perfect
Past Perfect Continuous VS Past Perfect
Present Perfect Continuous VS Present Perfect