Example: I have been reading this book since morning/three hours .(Reading action continuing, It can also mean that this action is not continuously ongoing because it may be a voluminuous book. It starts and stops at intervals. )
The Present Perfect Continuous is also used to show an action started in the past and may or may not be continuing now
Example: .I have been reading this book for the whole day .(Reading action may or may not be continuing, Here the action may not also be continuous. One could say I was reading the book for two hours in the morning (past continuous)and then after lunch I read it for one hour (simple past) and I am now reading the book.(present continuous))
Look how the past continuous expresses a past action at a definite time in the past.
So you can keep in mind that present perfect continuous expresses a continuing or ongoing state or condition while the past continuous expresses a completed past action at certain specific time.