past perfect continuous

use it

	This is a past tense, so it is about something that is over, is finished
	It is a past perfect tense, so it is about something that happened before another past action
	It is a continuous tense, so it is about something that was busy for some time in the past

The past perfect and the past perfect are really the same:
they tell you that one action in the past happened
before another action in the past.

The difference is not in what they express, but more in how they express it:

the past perfect gives the facts (first this, and then that)
He had waited for her and then she came.
the past perfect continuous stresses that the first action took a long time
He had been waiting for her all morning when she finally came.